Wash the currants and drain well. Strip the berries from the panicles. Chop the chocolate. Cut butter into pieces. Melt chocolate and butter in a saucepan over low heat. Mix flour, cocoa and baking powder.
Beat the eggs, brown sugar and salt in a bowl with the whisk of the hand mixer for about 5 minutes until creamy.
Remove the chocolate-butter mixture from the stove, immediately pour into the egg mixture and stir in. Quickly fold the flour mixture into the chocolate-egg mixture. Finally, carefully fold in currants and marshmallows.
Put the dough into a greased, square springform pan (24 x 24 cm). Bake in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 30 minutes. Remove and let cool on a cake rack.
Cut into pieces and decorate with currant panicles.