Start by browning 120g of the butter. To do this, chop the butter into evenly sized pieces then place into a medium saucepan. Place over a medium heat & allow to melt, stirring regularly with a spatula. Keep cooking, until the butter starts to foam, smells nutty & you can see that the milk solids in the bottom of the pan have turned golden brown.
Transfer the brown butter to a bowl & leave to cool. In the meantime,, chop the remaining 80g of butter into even pieces & leave to soften at room temperature.
Place both butters into a large mixing bowl & add both the brown sugar & the caster sugar. Cream the butter & sugars together with a wooden spoon until thoroughly combined. We don’t want to add too much air to the cookie dough at this point. 2-3 minutes of mixing should do the trick! In a separate bowl, lightly beat the egg & yolks together then gradually mix into the butter & sugar.
In another bowl, whisk together both the flours, espresso powder, salt, baking powder & bicarbonate of soda. Add to the butter mix & stir to combine.
Roughly chop 125g of the dark chocolate & all of the milk chocolate. Add the chopped chocolate to the cookie dough, stir to incorporate then chill the dough for an hour in the fridge.There should be a mix of larger pieces & some smaller pieces of chocolate. Whilst the cookie dough is chilling, chop the remaining dark chocolate into 15 equally sized chunks.
Once the dough is cool, scoop into 70g portions & roll into balls with your hands. Making sure that there’s plenty of chocolate in each portion. Press a dark chocolate chunk into the top of each ball.
Place the portioned cookie dough on a baking tray or into a large container then leave in the fridge overnight.