Preheat the oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.

In a large bowl, melt the butter. Stir in the sugar until combined, then mix in the eggs and vanilla extract.

In another bowl, whisk together the flour, cocoa powder, salt, and baking powder.

Gradually stir the dry mixture into the wet mixture until just combined.

Pour the brownie batter into the prepared baking pan, spreading it evenly.

Bake in the preheated oven for 25-30 minutes or until a toothpick inserted into the center comes out with a few moist crumbs.

Let the brownies cool completely in the pan.

For the buttercream, beat the softened butter in a large bowl until creamy.

Gradually add the powdered sugar, mixing until well combined.

Add heavy cream and vanilla extract, beating until fluffy. Adjust the consistency with more cream or powdered sugar as needed.

Once the brownies are cooled, spread the buttercream evenly over the top.

Top with sprinkles for a festive touch. Cut into squares and serve.