Preheat your oven to 350°F (175°C). Grease a 9x13-inch baking pan with butter or non-stick spray.

In a large mixing bowl, combine melted butter and sugar, whisking until smooth. Add eggs one at a time, beating well after each addition. Stir in vanilla extract.

In a separate bowl, whisk together flour, cocoa powder, and salt. Gradually add to the wet ingredients, mixing until just combined.

Divide the batter evenly into three bowls. Add red gel food coloring to one bowl and blue to another, leaving the third bowl plain.

Pour the plain batter into the prepared pan, spreading it evenly. Drop spoonfuls of the red and blue batters over the top, then use a knife to swirl them together.

Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out clean. Let cool completely in the pan on a wire rack.

In a medium bowl, beat softened butter until creamy. Gradually add powdered sugar, beating until smooth. Add milk and vanilla extract, mixing until combined and smooth.

Once the brownies are completely cool, spread the frosting evenly over the top. Use a spatula to create decorative swirls if desired.

Cut the frosted brownies into 16 squares and serve at room temperature. Perfect for parties or patriotic celebrations.

Store leftover brownies in an airtight container at room temperature for up to three days, or freeze for up to three months.