First, brown 120g of the butter, whisk in the toasted milk powder then leave to cool to room temperature, this should take around an hour to cool. Weigh your butter once browned, there should be 100g. 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 together. Add to the butter & sugar mix then beat together until combined.
Add the flours, espresso powder, salt, baking powder, bicarbonate of soda & cinnamon to the dough & mix until just combined.
Next, roughly chop 100g of the dark chocolate & all of the milk chocolate. Add to the cookie dough & stir in. Chill the dough in the fridge for 30 minutes to firm up.
Whilst the cookie dough is chilling, chop the remaining dark chocolate into 13 chunks.
Once the dough has firmed up, scoop into 13 equally sized balls, roughly weighing 80g each.Next, flatten a ball of cookie dough in your hands, place a marshmallow in the middle then wrap the cookie dough around the marshmallow & shape into a ball. Press a chunk of chocolate into the top.Repeat with the remaining cookie dough & marshmallows then place the cookies onto a baking tray/into a large container & refrigerate overnight. The next day, preheat an oven to 180°c/160°c fan.Place 3-4 balls of cookie dough onto a lined baking tray, making sure that there’s plenty of room between them. 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, baking 3-4 cookies at a time.