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, smell nutty & you can see that the milk solids in the bottom of the pan have turned golden brown.
Once the butter has browned, take it off the heat & immediately stir in the toasted milk powder. Transfer the butter to a bowl & leave to cool to room temperature. In the meantime, chop the remaining 80g of butter into small pieces & leave to soften at room temperature.The brown butter should take around 30 minutes to cool. Next, add both of the butters, the dark brown sugar & caster sugar to the bowl of a stand mixer with the paddle attached. Beat on a medium speed for 1-2 minutes, until combined.Be careful not to over mix the butter & sugar, we don’t want to add too much air into the cookie dough as this will affect the baked cookies. In a separate bowl, lightly beat the egg, yolks & vanilla together. Then slowly pour into the butter, with the mixer running on medium speed. Stop mixing once the eggs are just incorporated.
Next, sift the flour, baking powder, bicarbonate of soda & salt into the butter then mix with a spatula until just combined.
Roughly chop 100g of the dark chocolate & add into the cookie dough. Roughly chop the oreos then add into the cookie dough as well.
Using a spatula, stir the cookie dough, chocolate & oreos together then chill in the fridge for 30 minutes, to firm up slightly.
Whilst the cookie dough is chilling, chop the remaining 60g of dark chocolate into 13 chunks.
Using a cookie scoop (or a spoon), portion the chilled cookie dough into 13 equally sized portions. Each should weigh around 80 grams.
With your hands, roll a portion of cookie dough into a ball then press a dark chocolate chunk into the top. Repeat with the remaining dough.
Place the balls of cookie dough into an airtight container & refrigerate overnight.