I have never had much luck with cookies, but these (other than me getting distracted by Outnumbered on TV and not taking them out on time) seem to work well!
I used a recipe from http://www.goodtoknow.co.uk/recipes/524421/chocolate-cookies so thank you!
100g (3½oz) butter, softened
125g (4oz) light muscovado sugar
Yolk from 1 med egg
125g (4oz) plain flour
30g (1oz) cocoa powder
½ tsp bicarbonate of soda
100g bar dark chocolate (50-70% cocoa solids), roughly chopped
Cream the butter and sugar together until light and fluffy. Beat in the egg yolk, then sift in the flour, cocoa and bicarbonate of soda and mix well. Cool for 5 mins.
Add the chopped chocolate and nuts. Chill the mixture while the oven heats up to 190°C.
Divide the mixture into 10 and roll each portion into a ball. Arrange, spaced apart on the baking sheets and then press them to flatten them to about 1.5cm (½in) thick. Bake for 10-12 mins until risen and cracked but still a bit soft and gooey.
Cool for 5 mins, then transfer to a wire rack to cool.