Preheat the oven to 170°C and line 2 to 3 baking trays with baking paper. 

Melt the butter in a saucepan over medium heat. Cook until it foams, bubbles and turns lightly golden brown. Remove from the heat and pour into a mixing bowl, scraping in all the browned bits from the pan. Set aside to cool until just warm. 

Add the brown sugar and caster sugar, then mix until well combined and glossy. 

Add the egg, egg yolk and vanilla extract. Whisk until the mixture thickens slightly. 

Fold through the flour, bicarbonate of soda, salt and coffee powder until a dough forms. 

Add the chopped dark chocolate and pecans, then mix until evenly combined. 

Cover the dough and refrigerate for at least 3 hours, or overnight for the best flavour and texture. 

Roll the dough into rough balls weighing 80g to 90g each. Place on the lined trays, allowing at least 10cm between each cookie. 

Bake for 15 minutes, or until the edges are set and the centres are still slightly pale and soft. 

Remove from the oven, sprinkle with sea salt flakes and leave to rest on the tray for 5 minutes before serving.