Preheat your oven to 350°F (175°C). Grease and flour an 8x8 inch baking pan with gluten-free flour or line it with parchment paper.

In a medium bowl, whisk together the melted butter and 1.5 cups of granulated sugar until well combined.

Beat in the eggs one at a time, then stir in the vanilla extract.

In a separate bowl, whisk together the 0.5 cup gluten-free flour blend, cocoa powder, baking powder, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Do not overmix.

In a small bowl, whisk together the 0.5 cup granulated sugar, lemon juice, and lemon zest until dissolved.

Gradually whisk in the 0.5 cup gluten-free flour blend until the mixture is smooth and slightly thickened.

Pour half of the brownie batter into the prepared baking pan and spread evenly.

Drizzle the lemon swirl mixture over the brownie batter. Use a knife or toothpick to gently swirl the lemon mixture into the brownie batter, creating a marbled effect.

Pour the remaining brownie batter over the swirled mixture and spread evenly.

Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with moist crumbs (not wet batter).

Let the brownies cool completely in the pan before cutting into squares.