Preheat your oven to 350°F (175°C) and line a 9x13-inch baking pan with parchment paper.

In a mixing bowl, cream together softened unsalted butter and granulated sugar until light and fluffy, about 2-3 minutes.

Add the large eggs one at a time, mixing well after each addition, then pour in the vanilla extract and blend until fully combined.

In a separate bowl, whisk together the all-purpose flour, cocoa powder, baking powder, and salt until well combined.

Gradually add the dry ingredients to the butter mixture, mixing on low speed until just combined.

Spread the cookie dough evenly into the prepared baking pan.

Bake for 18 to 22 minutes until the edges are set and center appears slightly puffed.

Remove from oven and cool in the pan for 30-45 minutes.

Meanwhile, prepare the frosting by combining powdered sugar and milk or cream to achieve a smooth consistency.

Once cooled, frost the bars generously and sprinkle with chocolate or rainbow sprinkles.

Lift from the pan using parchment overhang, slice into bars, and serve.