Gel food coloring (I used Americolor gels Electric Pink, Electric Yellow, Electric Blue, Watermelon Red, Tulip Red, and Orange); piping bags; a large star piping tip (Wilton 2D is one of my favorites!); a bench scraper; an extra-tall cake scraper (optional).
Bake Your Cake:
Preheat your oven to 350°C Prepare four 6-inch-round, 3-inch-deep (6”x3”) cake pans by greasing with shortening, placing a baking paper round on the base, greasing again, and flouring.
In a medium-size bowl, combine your egg whites, whole eggs, 1/2 cup of your milk, and vanilla and whisk to combine. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment, combine your cake flour, sugar, baking powder, and salt and mix on low speed for a few minutes until combined.
Add in your butter and keep mixing. Stream in your remaining 1 1/2 cups milk, half a cup at a time, continuing to beat on the lowest speed. Raise the speed to medium and continue to beat for about 1 minute.
Scrape your bowl, and then return the batter to low speed. Add your egg mixture in three batches, allowing each to incorporate fully before adding the next. Scrape again and mix briefly.
Divide your cake batter into four portions. Tint the first portion a deep raspberry color using Watermelon Red gel color and Electric Pink with just a small touch of Electric Blue. Color the second portion sunset orange with Orange, a bit of Watermelon Red, and a bit of Electric Pink. Tint the third portion a deeper pink with Electric Pink and a touch of Watermelon Red, and the last portion a lighter pink with Electric Pink an a touch of orange.
Spoon your batter into your pans, swirling up to two colors together in each pan if you want a swirlier look, although you can also just do one color per pan!
Bake your cakes for roughly 30-35 minutes, but keep an eye on them starting at minute 25. You’ll know they’re baked through when you press down in the center of each cake and it springs back. You can also insert a cake tester, and it should come out completely clean.
Remove your cakes from the oven and allow to come to room temperature on a rack. Then, I like to wrap them up in plastic and put them in the fridge for at least half an hour, preferably overnight before I remove them from the pans. The cakes should be cold all the way through before attempting to trim them.
Prepare Your Buttercream:
Whip up some buttercream using the recipe linked in the ingredients list. You will need to make one and a half times the listed recipe.
Divide your buttercream into three equal portions. Set one of these portions aside.
Split one of the two portions you’re working with in half, and color one part fiery red using Americolor Tulip Red and the other part a peachy pink with Electric Pink, Watermelon Red, and the tiniest bit of Electric Blue.
Color the entirety of the second half a warm yellow using Electric Yellow and a touch of Tulip Red.
Divide the one portion of buttercream you’d set aside earlier into three smaller portions. Color each portion a different shade of orange to create an ombre effect that ranges from your yellow and your fiery red that you just finished mixing. Use Electric Yellow, Tulip Red, and Orange to achieve this.
Stack Your Base:
Level and torte each of your 4 cakes in half. Take three layers total and stack these first, spreading a thin layer of pink buttercream between the cake layers. Set in the fridge or freezer for about 10 minutes until the buttercream hardens up.
Now you’re going to create the rays of the sun in this section of cake. It’s tricky, so work patiently and take your time! Take your cake out of the fridge and, using a small, sharp paring knife, cut into the cake, inserting the knife halfway the side of the cake at about a 25° angle. Without using a sawing motion at all, pull the knife through the cake as you rotate it on your turntable, creating a cone shape that you’re cutting into your cake.
Pop the top of the cake out and set it aside. Spread a thin layer of yellow buttercream onto the exposed insides of the cake, then replace the top portion and gently press it into the buttercream to adhere. Place in the fridge or freezer for another 10 minutes to let it firm up again.
Repeat this step twice more, although the second time you’ll move your knife up to just inside the circumference of the top of the cake and hold it at a 45° angle, and the third time you’ll move your knife to a little over halfway along the radius of the top of the cake and hold it at a 65° angle. Make sure to rest the cake in the fridge or freezer between each cut to firm up the yellow buttercream!
Finally, flip that section of cake upside down and use that paring knife to carve a shallow dome into the exposed base of the cake. I used a spoon to press the cake down into a smoother dome shape after cutting.
Fill the dome completely with yellow buttercream and freeze or refrigerate again.
Begin stacking your remaining layers of cake on your cake board or plate, layering a small amount of pink buttercream between every cake layer. Reserve the final layer and set aside.
Grab your cake segment with the sun inside and flip it over onto the layers of cake you’ve been building up so that the sun dome is resting on the layers of cake below. Add a thin layer of pink buttercream on top of this segment and cap it with that final reserved layer of cake. Refrigerate for at least 10 minutes.
Decorate the Base:
Place your red and orange buttercreams into piping bags. Scoop about a half cup of your remaining yellow buttercream into a piping bag as well. Cut the tips off the bags to create a hole 1/8th of an inch wide on each.
Place the remainder of your yellow buttercream in a piping bag fitted with your large star tip and set aside.
Remove your stacked cake from the fridge and trim away the brown sides of the cake. Then, pipe your yellow buttercream in rings rising up the base of the cake until it reaches roughly 1/5th of the way up the sides.
Repeat with your ombre of orange buttercream, working your way up the sides of the cake, and finish with your red buttercream taking it all the way to the top, then covering the top of the cake entirely.
Use your bench scraper to gently smooth the sides of the cake, holding it completely still at the base while you spin your turntable, cleaning it off, then moving it up towards the top and repeating. You will create smooth stripes of color that barely blend into each other.
Smooth out the top of the cake with a small offset spatula, then, if you have one, use your extra tall cake scraper to smooth the sides one more time to create a perfectly cohesive smooth look. Tidy up the lip of buttercream at the top of the cake with your offset.
Pipe frilly poofs of yellow buttercream with your star tip at the top of the cake, then add your sprinkles.
Slice Your Cake:
Cut slices from your cake, either one at a time or, for a grand effect, remove an entire half of the cake at once to reveal the full sunset effect!