Wash the currants and drain well. Strip the berries from the panicles with a fork.
Grease a square springform pan (24 x 24 cm). Mix flour, cocoa and baking powder. Preheat the oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer).
For the dough, melt 250 g butter in a saucepan over a low heat. Break the chocolate into pieces, add and melt in the warm butter while stirring.
Beat the eggs, sugar and 1 pinch of salt in a large bowl with the whisk of the mixer for approx. 5 minutes until thick and creamy. Stir in the warm chocolate-butter mixture immediately. Stir in the flour mixture only briefly.
Finally, carefully fold in the currants and mini-marshmallows.
Fill the dough into the mould and smooth it down. Bake in a hot oven for about 30 minutes. The brownie should not be completely baked through, so there are still traces of dough on the wooden skewer when you make a stick test.
Take out and let it cool down on a cake rack. Remove from the mould and cut into pieces and decorate with currants if desired.