First, brown 120g of the butter over a medium heat, whisk in the toasted milk powder then transfer to a bowl. Leave to cool to room temperature, which should take around an hour to cool.
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 & vanilla paste 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 too much air into the dough as this will affect the baked cookie. In a separate bowl, lightly beat the egg & yolks together then with the mixer running on a medium speed, gradually mix into the dough.
Sift together the flour, baking powder, bicarbonate of soda, espresso powder & salt then add into the cookie dough. Mix on a low speed until just combined.
Roughly blitz half of the M&Ms in a food processor then add into the cookie dough along with the remaining whole M&Ms.If you haven't got a food processor, you could place the M&Ms into a sealed bag then break them up with a rolling pin. Next, roughly chop the peanuts & 35g of the dark chocolate & add into the dough. Mix on a low speed until combined. Chill the cookie dough in the fridge for 20 minutes, to firm up slightly.
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 90g each. Press a chunk of chocolate into the top.
Place the balls of cookie dough into a container (or onto a baking tray) then refrigerate overnight.