Preheat oven to 350°F. Line a 9-inch round cake pan with foil and spray with nonstick cooking spray.

Place butter and peanut butter in a large bowl. Using a hand or a stand mixer, mix until combined then add both sugars. Cream until smooth. Mix in egg, vanilla, and milk and mix on medium speed until combined.

Mix in baking soda and salt, then slowly mix in flour and mix just until the batter is smooth and comes together. Be sure to scrape the sides of the bowl during mixing.

Press batter into prepared pan. Use a fork to create criss-cross marks all over the top.

Bake for 15-20 minutes or just until golden brown. Cool completely.

Make frosting: whisk butter, cocoa, powdered sugar, salt, and 1 tablespoon milk until smooth, adding a bit more milk as needed for consistency. Place frosting immediately in piping bag and frost immediately, top with sprinkles.

Store loosely covered for up to 2 days, longer in refrigerator.