Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.

In a large bowl, whisk together flour, baking powder, baking soda, and salt.

In a separate bowl, cream together sugar and butter until light and fluffy. Beat in eggs one at a time, then stir in vanilla.

Gradually add dry ingredients to wet ingredients, alternating with buttermilk, beginning and ending with dry ingredients. Mix until just combined.

Pour batter into prepared pan and bake for 35-40 minutes, or until a wooden skewer inserted into the center comes out clean.

Let cake cool completely before frosting.

In a large bowl, beat butter until smooth. Gradually add powdered sugar, beating until combined.

Add milk and vanilla extract and beat until light and fluffy.

Frost the cooled cake.