Your vanilla cake mix – make batter according to recipe instructions. Add the 1 teaspoon yellow gel color to mix. Spray a baking sheet with vegetable oil or cooking spray; fit a piece of parchment paper on top. Add cake batter and bake for ten to twelve minutes, until set but still underbaked.
With clean hands, roll underbaked cake batter into 1-inch cake balls.
Place in freezer. Now, make the white cake batter.
Preheat oven to 350 degrees F.
Spritz a 9 x 9 inch square cake pan with vegetable oil and line bottom with parchment paper square cut to size.
In a large bowl, sift together the cake flour, baking powder and salt and set aside.
In the bowl of a stand mixer fitted with the paddle attachment, cream the butter and sugar for two minutes on high speed, scraping down sides of bowl with spatula. With the mixer on low, add the egg whites one at a time until incorporated, then add vanilla extract.
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.
Remove the yellow cake balls from the freezer. Place cake balls into the square cake pan, distributing them evenly.
Carefully pour white cake batter into the pan on top of the cake balls, covering the cake balls with batter, about 2/3 full. Any leftover batter can be used to make cupcakes.
Bake for 22-25 minutes, until a toothpick inserted into the middle of the cake comes out only with moist crumbs. Let cake cool completely before frosting!
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter, icing sugar and vanilla on high speed until it is light and fluffy and doubled in volume, about three minutes.
Using an offset spatula, frost the cake with the vanilla buttercream, using a cake bench scraper to smooth the frosting. Place in freezer to set for 12 minutes, then do a final clean coat of buttercream.
Assembly & Decoration:
Using clean and dry hands, shape a 3 inch ball of yellow fondant into a dripping egg yolk with tiny arms. Hand shape a few egg yolk drips and set aside.
Roll out some white fondant and using a parking knife, cut out sharp egg shell type pieces to decorate the sides of the cake. I also made an egg by cutting out the fondant with a small circle cutter and cutting zig zag edges into it to look like a broken egg. Use a small amount of white to make a “reflection” for Gudetama’s face. Adhere with a tiny amount of water to stick.
Using a tiny amount of black fondant, make the facial features for Gudetama – black eyes and a little round circle mouth. Use a toothpick tip to add a tiny amount of water to the yolk to stick the black pieces on top.
Place Gudetama onto the cake, draping his drippy legs over the side. Add the yolk drips by pressing them into the cake.
Add the egg shell pieces all along the edges of the cake.
Slice into the cake to reveal the “egg yolk” cake slices!