Mix the flour, cocoa, baking powder and vanilla in a bowl. Melt the butter and chop 5.5 oz (150 g) chocolate in a water bath while stirring. Separately, beat the eggs and sugar into a fluffy cream and carefully add the chocolate mixture, which has cooled by now.
Stir gently and add this mixture to the flour. Stir with a wooden spoon until you get a thick and sticky dough. Finally, add the chocolate chips/crushed chocolate. Use 1 tablespoon of dough for each biscuit and place the biscuits in an oven dish with baking paper, some distance apart from each other.
Put the biscuits to bake for about 13 minutes in a 338°F (170 °C) oven. Be careful not to overcook them, because they have to remain moist. They are ready once their tops crack and the surface becomes rough.