01 - Preheat oven to 350°F. Grease and flour a 9x13-inch baking dish.
02 - Whisk together flour, baking powder, baking soda, and salt in a medium bowl.
03 - Beat butter and sugar with an electric mixer until light and fluffy, approximately 3 minutes.
04 - Add eggs one at a time, beating thoroughly after each addition. Mix in vanilla extract.
05 - Alternate adding flour mixture and buttermilk to butter mixture, beginning and ending with flour. Mix until just incorporated.
06 - Spread batter evenly in prepared pan. Bake for 30-35 minutes until a toothpick inserted in the center emerges clean.
07 - Allow cake to cool in pan for 15 minutes, then transfer to a wire rack to cool completely.
08 - Whisk powdered sugar, milk, and vanilla until smooth and pourable. Drizzle over cooled cake and let set before slicing.