Preheat the oven to 180°C fan.

Add the melted butter and sugars to a bowl and beat until creamy.

Add the egg and beat again until smooth.

Sift in the flour, baking powder and bicarbonate of soda, then mix until a cookie dough forms. If it’s too dry and crumbly then just add a tiny splash of water and continue to mix.

Add the chopped chocolate and walnuts, mix until evenly distributed.

Divide the dough into 8 equal balls, about 125g each.

Place 4 cookies per lined baking tray, leaving space between them.

Bake for 14 minutes until the edges are set but the centres are still soft. Don’t flatten them.

Leave to cool on the tray for at least 10 minutes, then transfer them onto a cooling rack, they’ll firm up as they cool.