Preheat oven to 180 C (350 F). In a bowl, sift together flour, baking powder, dash of salt and lemon rind.

Melt butter and chocolate together in a double boiler over low heat, stirring continously until smooth. Let it cool.

Add sugar and eggs, one at a time, beating until fluffy. Add flour mixture to butter mixture. Add lemon juice. Stir until all ingredients are evenly combined.

Pour batter into prepared baking pan, a 20 cm rectangular pan with baking paper. Bake in preheated oven for 25-30 minutes.

Remove from the oven and let it cool. Sprinkle powder sugar before serving.