Weigh 120g of plain flour, 30g of cocoa powder and 1½ tsp of baking powder and set aside. Line a baking sheet with baking parchment. Chop 100g of dark chocolate into small pieces, setting 35g of it aside. Melt the larger quantity of chocolate in a bowl resting over a pan of simmering water. Avoid the temptation to stir. As soon as the chocolate is melted, remove from the heat. Put 50g of butter in the bowl of an electric mixer, add 150g of soft brown or light muscovado sugar and beat until you have a thick creamy paste. Break 1 egg into a small bowl or cup and whisk lightly with a fork, then add to the butter and sugar with 1 tsp of vanilla extract. Gently stir in the melted chocolate. Gently but thoroughly fold the flour and cocoa into the chocolate mixture, then the reserved chocolate pieces. Set the dough aside in the fridge for a good 25 minutes. Reduce 65g of salted, roasted peanuts to crumbs in a food processor, then scatter on a plate. Set the oven at 180C/gas mark 4. Remove the chilled dough from the fridge and scoop out about 15 balls (about 3 tsp each). Each ball should weigh about 30g. Roll the balls of dough in the crushed, salted peanuts, then place on a baking sheet, giving each room to spread. Bake for about 8 minutes until the biscuits are puffed and risen (the centres should still be soft and fudgy), then gently press each cookie with a dessert spoon, using just enough pressure to crack the surface. Dust lightly with a few sea salt flakes and leave to settle for 5 minutes before carefully transferring to a cooling tray using a palette knife. Continue with the rest of the mixture.