Preheat oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper and lightly grease.
In a large bowl, whisk together the melted butter and granulated sugar until smooth and glossy.
Add the egg, watermelon extract, vanilla extract, and pink gel food coloring (if using). Mix until fully combined and evenly pink.
Stir in the flour, baking powder, and salt. Mix just until combined — do not overmix.
Add the milk and stir until the batter is thick and smooth.
Spread the batter evenly into the prepared pan.
Bake for 22–25 minutes, or until the edges are set and the center looks slightly soft but not wet.
Allow the brownies to cool completely in the pan before adding glaze.
Optional Watermelon Glaze: In a small bowl, whisk together powdered sugar, milk (start with 1 tablespoon), watermelon extract, and a tiny drop of pink food coloring until smooth. Adjust milk as needed for a pourable consistency. Spread evenly over cooled brownies.
Let the glaze set for 20–30 minutes before slicing and serving.