For the cakes: You'll need flour, baking powder, baking soda, salt, butter, sugars, egg, vanilla, and buttermilk.
For the pastry cream: You'll need egg yolks, sugar, butter, cornstarch, milk, and vanilla.
Make the batter
Cream the butter and sugars together. Mix in the egg and vanilla. Alternate adding the dry ingredients and the buttermilk.
Pipe the batter onto a baking sheet and bake at 375 degrees F for about 10 minutes.
Cool on wire rack
Allow the little cakes to cool on a wire rack.
Top with chocolate ganache
Make the chocolate ganache and spread on the top cakes.
Fill with pastry cream
Spread the pastry cream on the bottom paired cakes.
Assemble the whoopie pies and enjoy!