Cream the butter and sugars together until soft and fluffy (around 3
minutes with an electric hand mixer)
Beat in the egg, followed by the vanilla extract.
Fold in the sifted flour, cocoa, baking powder, bicarb and salt,
followed by the chocolate chips. Chill the dough for at least 2 hours
Whilst the dough chills, make the crumble topping. Cut the butter
into cubes and rub the butter into the flour and sugar until you
have a mixture that resembles breadcrumbs. Stir the peanuts in.
When ready to bake, preheat the oven to Gas 4/180C and line
baking trays. Roll cookie dough into balls and press the crumble
topping into the dough.
Bake cookies for exactly 8 minutes (no longer). They will appear
underdone, but will firm up as they cool. Leave on the baking sheet
for 10 minutes and move to a wire rack to finish cooling.