Make the Cake

1. Preheat oven to 350º and prepare a lined sheet pan
2. Beat butter, sugar and vanilla in mixer for few minutes.  Meanwhile sift together your dry ingredients and set aside.  Add egg to butter mixture and beat until well incorporated
3. Add dry ingredients to butter mixture, in three additions, alternating with milk, starting and ending with dry ingredients.  Make sure to scrape down mixing bowl after adding milk
4. Portion out batter with ice cream scoop or pastry bag onto prepared sheet pan, allowing for 1-2" between, and bake for 8-10 minutes, or until the tops of cakes bounce back when pressed. Cool completely (preferably made day in advance) before filling

Make the Buttercream & Assemble Whoopie Pies

1. Cream together butter and vanilla with beater attachment.  Slowly add powdered sugar on low speed until all incorporated.  Add milk to thin out buttercream to desired consistency if necessary. Fill whoopie pies and enjoy!