1. Preheat the Oven:
   Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking pan with butter or line it with parchment paper for easy removal.

2. Prepare the Brownie Base:
   In a large mixing bowl, whisk together the melted butter and granulated sugar until smooth. Add the eggs one at a time, whisking well after each addition. Stir in the vanilla extract.

3. Add Dry Ingredients:
   In a separate bowl, sift together the flour, cocoa powder, salt, and baking powder. Gradually add the dry ingredients to the wet ingredients, stirring until fully combined.

4. Bake the Brownies:
   Pour the brownie batter into the prepared pan and spread it evenly. Bake for 25-30 minutes or until a toothpick inserted into the center comes out with only a few crumbs. Allow the brownies to cool completely in the pan on a wire rack.

5. Prepare the Frosting:
   In a medium-sized bowl, beat the softened butter until smooth. Gradually add the powdered sugar, milk, and vanilla extract. Mix until smooth and creamy. If you want a chocolate frosting, add 1 tablespoon of cocoa powder and mix until combined.

Frost the Brownies:
Once the brownies have cooled completely, spread the frosting evenly over the top using a spatula. Slice into squares and serve!