Preheat oven to 180°C (350°F).

Grease and line an 8-inch square baking pan with parchment paper.

Melt butter and dark chocolate in a heatproof bowl over a saucepan of simmering water, or in the microwave.

In a large bowl, beat sugar and eggs until light and fluffy.

Stir in vanilla extract.

Pour melted chocolate mixture into the egg mixture and mix well.

In a separate bowl, whisk together gluten-free flour, cocoa powder, and salt.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Pour batter into the prepared baking pan.

Bake for 20-25 minutes, or until a toothpick inserted into the center comes out slightly fudgy.

Let cool completely before slicing and serving.