Weigh 120g of the butter into a medium sized saucepan then place over a medium heat. Cook, stirring regularly until brown then stir in the toasted milk powder.Transfer to a bowl, leave to cool then refrigerate until the butter is just starting to firm up. Whilst your brown butter is cooling, chop the remaining 80g of butter into even pieces & leave to soften at room temperature.
Add both of the butters & the sugars to the bowl of a stand mixer with the paddle attached. Beat until combined & just turning pale, 1-2 minutes. Don’t overmix, we don’t want to add in air to the cookie dough as this will affect the baked cookie. In a separate bowl, lightly beat the egg, yolks & vanilla essence together. Add to the butter & sugar mix then beat together until combined.
In another bowl, whisk together both the flours, espresso powder, ginger, salt, baking powder & bicarbonate of soda.
Add to the butter mix & beat on a low speed to combine.
Next, roughly chop 130g of dark chocolate & 50g of white chocolate. Add to the cookie dough & stir in. Chill the dough in the fridge for 30 minutes to firm up. Whilst the dough is in the fridge, chop both remaining chocolates into 15 equally sized pieces (15 pieces of dark & 15 pieces of white).
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 both a dark & white 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.
The following day, preheat your oven to 180°c/160°c fan (355°f/320°f).
Line 3 large baking trays with baking parchment & place 5 cookies on each tray, making sure there’s plenty of room between them.For best results, bake the cookies one tray at a time. Bake the cookies for 8 minutes then open your oven door, lift up the baking tray inside the oven, by a few inches then let it drop down against the oven rack. Use enough force so that the cookie starts to deflate. (This is the “Pan Banging” method.)
Bake for a further 2 minutes then repeat the pan banging process.
Bake for another 2 minutes (bringing the total bake time to 12 minutes) then remove from the oven.
Using a large, round cookie cutter, scoot each cookie into a perfect round shape (this step is optional but makes the cookies more uniform).
Sprinkle each cookie lightly with Maldon salt, leave to cool slightly then transfer to a wire rack to cool completely.
Repeat the cooking process with the remaining cookies.