01 - Preheat oven to 350°F. Grease and flour a 9x13-inch baking pan.
02 - In a medium bowl, whisk together flour, baking powder, baking soda, and salt. Set aside.
03 - In a large bowl, cream butter and sugar until light and fluffy, about 3 minutes.
04 - Add eggs one at a time, beating well after each addition. Stir in vanilla extract.
05 - Alternately add dry ingredients and buttermilk to the butter mixture, beginning and ending with dry ingredients. Mix until just combined.
06 - Pour batter into prepared pan and smooth the top. Bake for 28–32 minutes, or until a toothpick inserted in the center comes out clean.
07 - Let cake cool completely in the pan on a wire rack.
08 - Beat together powdered sugar and butter until well combined. Add vanilla and milk, one tablespoon at a time, until smooth and spreadable.
09 - Spread frosting evenly over the cooled cake. Slice and serve.