1. Preheat oven to 350°F (175°C) and line a 9×13-inch pan with parchment paper.

2. Melt butter in a microwave-safe bowl or saucepan until smooth, then whisk in sugar.

3. Add eggs one at a time, whisking thoroughly after each addition to create a glossy batter.

4. Sift together flour, cocoa powder, baking powder, and salt, then fold into the wet mixture until just combined.

5. Fold in optional mix-ins such as white chocolate chips, marshmallows, or patriotic M&Ms if desired.

6. Pour batter into the prepared pan and smooth the top with a spatula.

7. Bake for 25–30 minutes until a toothpick inserted near the center comes out with moist crumbs for fudgy brownies.

8. Let brownies cool in the pan for 10 minutes, then transfer to a cooling rack and cool completely.

9. Spread a thin layer of white frosting or melted white chocolate over the cooled brownies.

10. Decorate with red, white, and blue sprinkles, candies, or edible glitter, arranging stripes or star patterns for a dessert flag effect.

11. Use a sharp knife to cut into 16 squares and serve on a festive platter or in individual liners.