Preheat the oven to 350°F. Line a light-colored metal 8x8-inch baking pan with parchment paper, leaving an overhang.

Melt the butter in a large microwave-safe bowl or saucepan. Whisk in the sugar, then cool to just warm before whisking in the eggs, egg yolk, and vanilla until smooth. Stir in the flour, baking powder, and salt until just combined.

Press the dough evenly into the prepared pan using a spatula.

Bake for 30-35 minutes, or until lightly golden brown and the center springs back. Cool completely on a wire rack.

To make the frosting, beat the butter in a stand mixer until creamy, about 2 minutes.

On low speed, add the powdered sugar, cocoa powder, cream, vanilla, and salt. Increase speed to high and beat for 3 minutes until light and fluffy.

Remove the cooled bars from the pan using the overhang. Frost with a thick layer of frosting.

Sprinkle with sprinkles before cutting into squares. Serve or store in an airtight container.