Sift the flour, cocoa and bicarbonate of soda into a bowl.

In a separate bowl, mix together the melted butter, brown sugar and caster sugar. Add the egg and egg yolk and beat well together. Then add the sifted ingredients and peanuts and stir until thoroughly combined.

Very roughly chop the Cadbury Flake 99 into chunks and lightly stir into the cookie dough.

Spoon mounds of dough (about the size of a golf ball for medium sized cookies) onto baking sheets lined with non stick baking paper. Allow plenty of space for the cookies to spread, so only cook 5 or 6 per baking sheet and cook in batches if necessary.

Bake at 180 °C/160 °C fan/gas mark 4 for about 12 minutes for medium sized cookies or until the edges are just beginning to brown. Larger biscuits may take 2-3 minutes more. Allow to cool before serving. Or store in an airtight container for up to three days.