Gel color in red, pink, yellow, green, purple, blue.
Piping bags each fitted with a variety of different piping tips such as an open star tip, a grass tip, etc. I used Wilton 1M, Wilton #234, Wilton #4B, and an open circle tip.
5 medium bowls (you can use large cereal bowls or a mixture of whatever you have!).
To Make The Cake:
Spritz the cake pans with vegetable oil and line with parchment paper circles cut to size. Preheat oven to 350 degrees F.
In a large bowl, sift together the cake flour, baking powder and salt.
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugar on high speed until light and fluffy, about two minutes.
With the mixer on low speed, add the eggs one at a time, gently incorporating. Add the vanilla extract.
Whisk together the cream and milk.
With the mixer on low speed, add the dry flour mixture in three additions, alternating with the milk and ending with the flour mix, about one minute in total, until batter is smooth and free of lumps and bumps. Do not overmix.
Divide the batter evenly into the five medium bowls. Add a different color of gel food coloring to each bowl, mixing gently to combine. To get a bright shade of red, you will need more red gel color than the other colors. Start with less and add more as needed.
Pour each colored batter into its own separate cake pan. Gently rap the cake pan a few times on the counter to discourage air bubbles. Bake for 15-20 minutes, until cake pulls away from the sides of the pan and a toothpick inserted in the center comes out with moist crumbs. Let cool completely on wire racks before frosting with buttercream.
To Make The Buttercream and Frost The Cake:
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, icing sugar, vanilla and tiny pinch of salt on high speed until light and fluffy and doubled in volume, about three minutes.
With the cake layers completely cool, fill and frost the cake with the plain white vanilla buttercream, stacking the layers following the ROYGBIV colors. If you want the red layer to be on the top, start with the purple layer, top with frosting, then stack the blue, then green, yellow and finish with the red. Frost and coat the entire exterior of the cake, using a cake bench scraper to smooth as you go.
Divide the remaining buttercream into five or six bowls, depending on what shades you want for your rainbow piping.
Add ¼ teaspoon of gel color to each bowl to start, mixing in the color to get the desired shade. Again, with the red gel color, you will need more gel color to get that vibrant red.
Fill your piping bags fitted with the various piping tips each with a different buttercream color. Pipe rosettes, drop stars and furry bits of grassy buttercream all over the cake. I start on the top and pipe the top of the cake first, making sure my colors and patterns are evenly dispersed! I tend to work with one color, piping all over, then switching colors, then adding more colors and then filling in any naked spots at the end.
Tip: for a sharp, clean cut – chill your cake for 10 minutes in the freezer first before slicing!