In a large bowl, using an electric/stand mixer (or hand whisk, just double/triple the times) combine the butter and both sugars on medium speed for several minutes, until fully combined. Add the egg and vanilla and mix again.
Add the flour, baking powder, chocolate and walnuts and fold until combined. The dough should be fairly solid at this stage (it ought to almost form a ball, albeit a very soft one!).
Cover and chill in the fridge at least 30 minutes.
Preheat the oven to 200C/180C Fan/Gas 6. Line 2-3 large baking trays. Roll into about 14 balls (use a tablespoon to scoop balls of dough). Place on lined baking sheets. Bake 10-12 minutes, until just golden at the edges. Let cool on a wire rack before eating!
