Grease a square springform pan (approx. 24 x 24 cm; alternatively a round one of 26 cm Ø) and dust with flour. Coarsely chop the pistachios and almonds. Caramelise 150 g sugar in a wide pot while stirring constantly until golden brown.
Add cream and simmer at low heat until the caramel has dissolved in the cream. Remove from heat. Preheat the oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer).
Break chocolate into pieces. Melt with the butter in a hot water bath, let it cool down a little. Beat the eggs and 350 g sugar with the whisk of the mixer for about 10 minutes until thick and creamy. Stir in the syrup.
Mix flour, baking powder and 1 pinch of salt. First stir the chocolate mixture into the egg mixture, then the flour mixture, then 30 g each of almonds and pistachios.
Smooth the dough in the form. Bake in a hot oven for 50-60 minutes, possibly covering with aluminium foil about 10 minutes before the end of the baking time. Take out the cake and prick several times with a wooden skewer.
Drizzle with caramel sauce and sprinkle with remaining almonds, pistachios and marshmallows. Bake the cake in a hot oven at the same temperature for another 5 minutes. Let them cool down.