Grease the baking tin (approx. 18 x 39 cm) or half of the fat pan and dust with flour (delimit the fat pan with a strip of multi-folded aluminium foil). Divide 300 g chocolate into pieces, melt with butter in the pan. Let cool for about 5 minutes. Grate apple. Weigh 150 g fruit flesh.
For the dough mix flour, almonds, cocoa and baking powder. Mix eggs, sugar and 1 pinch of salt in a mixing bowl with the whisks of the mixer for about 5 minutes until creamy. Stir the liquid chocolate into the egg mixture. Stir the flour mixture and the grated apple one after the other into the dough.
Spread the dough into the mould. Bake in the preheated oven (electric cooker: 175 °C/circulating air: 150 °C/gas: see manufacturer) for approx. 25 minutes. Let it cool down.
For the icing, divide 200 g of chocolate into pieces. Melt with cream and coconut oil in a pot. Let it cool down for about 5 minutes. Spread liquid chocolate on the brownie. Keep cool until serving.