Step 1: Beat butter and brown sugar on medium speed until light and fluffy.
Step 2: Add egg yolks and vanilla. Beat until fully combined.
Step 3: Combine flour, cocoa powders, baking powder, and salt in a bowl and stir with a whisk. Add to butter mixture and beat until combined.
Step 4: Using a cookie scoop, place scooped cookie dough on parchment lined baking sheet. Chill for 40 minutes.
Step 5: Bake in 350° oven for 11-13 minutes. Remove from oven and cool completely.
Step 6: Beat butter at medium speed for 4 minutes.
Step 7: Add powdered sugar and vanilla. Beat at medium speed for 4 minutes.
Step 8: Add whipping cream and beat for additional 2 minutes.
Step 9: Spread 1-2 tablespoons of frosting on each cookie.
Step 10: Decorate with your favorite sprinkles.
Step 11: Enjoy!!