Preheat oven to 350 degrees F (175 degrees 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 butter and sugar until light and fluffy. Beat in eggs one at a time, then stir in peanut butter and 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 20-25 minutes, or until a wooden skewer inserted into the center comes out clean.

Let cool completely before frosting.

In a medium bowl, beat butter until creamy. Gradually add powdered sugar, peanut butter, milk, and vanilla extract. Beat until smooth and creamy.

Frost the cooled cake.