Stir together the flour 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, walnuts, Crunchie Bits and stir until thoroughly combined.

Allow the mixture to chill and firm up before spooning 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 160 °C fan, 180 °C conventional or 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.