01 - Preheat oven to 350°F. Grease and flour a 9x13-inch baking dish thoroughly.
02 - Whisk together flour, baking powder, baking soda, and salt in a medium bowl until evenly blended.
03 - Beat softened butter and granulated sugar with an electric mixer on medium-high speed until pale and fluffy, approximately 3 minutes.
04 - Add eggs one at a time, beating thoroughly after each addition. Mix in vanilla extract until fully incorporated.
05 - Alternately add flour mixture and buttermilk to butter mixture, beginning and ending with flour mixture. Stir until just combined, being careful not to overmix.
06 - Spread batter evenly into prepared baking pan, smoothing the surface with a spatula.
07 - Bake for 30-35 minutes until a wooden toothpick inserted into the center emerges clean or with just a few moist crumbs.
08 - Let cake cool in the pan for 15 minutes, then carefully transfer to a wire rack to cool completely before glazing.
09 - Whisk powdered sugar, milk, and vanilla extract until smooth and drizzle-consistency. Add additional milk if needed to achieve desired flow.
10 - Drizzle glaze evenly over the completely cooled cake. Allow glaze to set for at least 15 minutes before slicing and serving.