Step 1

Make the vanilla cake:

Step 2

Heat the oven to 350 F.

Step 3

Combine the butter and sugars in the bowl of a stand mixer fitted with the paddle attachment and cream together on medium-high for 2 to 3 minutes. Scrape down the sides of the bowl, add the eggs and mix on medium-high again for 2 to 3 minutes. Scrape down the sides of the bowl once more.

Step 4

On low speed, stream in the buttermilk, oil and vanilla. Increase the mixer speed to medium-high and paddle for 4 to 6 minutes, until the mixture is practically white, twice the size of your original fluffy butter-and-sugar mixture, and completely homogenous. Don't rush the process. You're basically forcing too much liquid into an already fatty mixture that doesn't want to make room for the liquid. Stop the mixer and scrape down the sides of the bowl.

Step 5

On very low speed, add the cake flour, baking powder and salt. Mix for 45 to 60 seconds, just until your batter comes together and any remnants of dry ingredients have been incorporated. Scrape down the sides of the bowl. If you see any lumps of cake flour in there while you're scraping, mix for another 45 seconds.

Step 6

Spray a quarter sheet pan with nonstick cooking spray and line it with parchment, or just line the pan with a Silpat. Using a spatula, spread the cake batter in an even layer in the pan. Give the bottom of your sheet pan a tap on the countertop to even out the layer.

Step 7

Bake the cake for 30 to 35 minutes. The cake will rise and puff, doubling in size, but will remain slightly buttery and dense. At 30 minutes, gently poke the edge of the cake with your finger; the cake should bounce back slightly and the center should no longer be jiggly. Leave the cake in the oven for an extra 3 to 5 minutes if it doesn't pass these tests.

Step 8

Take the cake out of the oven and cool on a wire rack or, in a pinch, in the fridge or freezer (don't worry, it's not cheating). The cooled cake can be stored in the fridge, wrapped in plastic wrap, for up to 5 days.

Step 9

Make the mint frosting:

Step 10

In the bowl of a stand mixer fitted with the paddle attachment, combine powdered sugar, cornstarch, salt and shortening on low speed for 1 minute, then on medium high speed for 4 minutes until no lumps remain.

Step 11

Scrape the bowl. Mix on high speed for an additional 2 minutes. Mixture will be sandy.

Step 12

Add corn syrup, clear vanilla extract and milk, and mix on low speed for 1 minute, then high speed for 2 minutes.

Step 13

Scrape bowl. Mix on high speed for 1 minute.

Step 14

Add mint flavor and green food coloring. Mix on low speed for 1 minute then high speed for 2 minutes.

Step 15

Scrape the bowl. Mix on high speed for 1 minute.

Step 16

Add dark chocolate flakes and mix on speed 1 for 1 minute, then speed 3 for 30 seconds until pieces are broken and distributed.

Step 17

To assemble:

Step 18

Let cake cool completely before frosting. Apply frosting with an offset spatula for a beautifully smooth finish.