Put the chopped butter in a pot and start heating over low heat. Once it begins to melt, add 150 g of the chocolate, broken into small pieces, and stir until fully melted. Remove from heat.
Beat the eggs with the sugar for about 5-7 minutes until fluffy.
Add the chocolate mixture and stir, or mix slowly, until smooth. Add the flour and baking powder and combine again.
Sprinkle in the remaining chocolate (50 g), finely chopped (and the chopped nuts, if using). Place in the freezer for 10 minutes (or in the fridge for about half an hour).
Preheat the oven to 180°C.
Line a large baking sheet with parchment paper.
Scoop the batter by the spoonful onto the baking sheet, spacing them apart (no need to flatten the dough). Bake for 10 minutes. When removed from the oven, the cookies will be soft in the middle; they firm up only after cooling.
