Melt butter and dark chocolate using a double boiler or microwave, then let it cool. Stir gently with a spatula.

Whip the egg, sugar, and salt until slightly frothy.

Add the melted butter and chocolate mixture to the egg mixture and combine.

Sift rice flour, cocoa powder, baking powder, and baking soda into the mixture and fold lightly.

Wrap the dough in plastic wrap and refrigerate for 1-2 hours.

Shape the dough into 150g balls, roll them in powdered sugar, and place them on a baking tray.