Preheat oven to 350 degrees Fahrenheit. Grease a 13x9 baking pan by spraying with cooking spray. Set aside.
In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.
With an electric mixer, cream butter and sugar together until fluffy (1-2 minutes).
Add eggs one at a time, mixing after each addition.
Add vanilla. Then, with mixer running on low, alternately add milk and dry ingredients. Mix until just combined.
Pour batter into the prepared pan and bake 35-45 minutes until the center of the cake springs back when you touch it.
Let the cake cool completely before frosting.
Make the frosting:
In a stand mixer bowl, with the mixer on medium speed, whip the butter for 2 minutes until it is almost white in color. Make sure the butter is at room temperature, but not melted.
Add in the powdered sugar, vanilla and 3 tablespoons cream. Beat on low speed for 3-5 minutes, adding in the last tablespoon or two of milk if the buttercream needs to be thinner.
Fold in the sprinkles.
Spread on cooled cake and sprinkle with additional sprinkles if desired.
Notes
The calories shown are based on the cake being cut into 15 pieces, with 1 serving being 1 slice of cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate. **We are not dietitians and recommend you seek a nutritionist for exact nutritional information. The information in the nutrition box are calculated through a program and there is room for error. If you need an accurate count, I recommend running the ingredients through your favorite nutrition calculator.**