Step 1

FOR THE CAKE:

Step 2

Heat the oven to 350°.

Step 3

Coat an 8" square cake pan with nonstick spray and line the bottom with parchment paper.

Step 4

Beat the butter and sugar together in a large bowl with an electric mixer until fluffy.

Step 5

Beat in egg, yolks, and vanilla until combined.

Step 6

Scrape down the bowl, and then sprinkle the baking powder, baking soda, and salt over the batter and beat until very well combined.

Step 7

Add the buttermilk and mix to combine (the batter may look curly).

Step 8

Add the cornstarch and flour and mix until the flour just disappears.

Step 9

Scrape the batter into the prepared pan and smooth the top.

Step 10

Bake for 25 to 27 minutes, until a toothpick inserted into the center of cake comes out batter-free.

Step 11

Let cool in the pan on a cooling rack for 10 minutes, then run a knife around the cake to ensure that it is loose.

Step 12

Flip in out onto a cooling rack, and then flip back right side up again onto another.

Step 13

Let cool completely before frosting.

Step 14

FOR THE FROSTING:

Step 15

Beat the butter, powdered sugar and salt in a large bowl with electric beaters (or paddle attachment for stand mixer) until fluffy.

Step 16

Pour in the chocolate, cream or milk, and vanilla and beat until combined, then 1 more minute to whip it further.