Preheat oven to 170°C. Grease and line a baking pan.

Melt butter and chocolate chips together (double boiler or microwave) until smooth.

Whisk in sugar, then eggs one at a time, then vanilla.

Sift in flour, cocoa powder, and baking powder; fold until just combined.

Fold in most of the chopped nuts, reserving some for topping.

Pour into the pan, sprinkle remaining nuts on top.

Bake 25-30 minutes until set but still fudgy in the center.

Cool in the pan before slicing.