Preheat oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer). Grease square springform pan (24 x 24 cm; alternatively round, 26 cm Ø). Break chocolate into pieces for the dough.
Melt with 250 g butter in pieces in a saucepan at low heat.
Beat the eggs, sugar and 1 pinch of salt with the whisks of the mixer for 3-4 minutes until creamy. Remove the chocolate butter from the heat and immediately stir into the egg mixture.
Mix flour, cocoa and baking powder. Quickly stir into the chocolate-egg mixture.
Cut marshmallows in half. 1⁄3 Spread the dough in the mold. 2⁄3 Put marshmallows on top. Add the rest of the dough. Spread the rest of the marshmallows on top and press in gently. Bake in a hot oven for about 25 minutes.
Remove and let it cool down. Remove from the mould and cut into approx. 4 x 4 cm pieces.