To make the brown butter: Place sliced butter ( butter is sliced so that it melts faster and evenly) in a saucepan over low heat. Slowly melt the butter until it is foamy on the surface. Pour the melted butter into a heatproof or glass bowl. Let it stand for 1 minute. Use a teaspoon and carefully remove the foam from the surface. Return the melted butter to the saucepan and heat until light brown specks begin to form and you smell a nutty aroma. Remove from heat and and pour into a small bowl with all the bits. Leave it to cool for 20 minutes and then place in the fridge to chill for 1 hour.
Sift flour, baking soda and salt in a large mixing bowl. Set aside.
In the bowl of a stand mixer, beat together brown butter, brown sugar and granulated sugar . Add whole egg, then yolk and vanilla extract on low speed and beat until well blended.
Add flour, malted milk powder and beat on low until just incorporated. Mix in milk. Fold in the chocolate chunks.
Cover the cookie dough and chill in the fridge for at least 2 hours or overnight. ( The cookie dough will be quite sticky at this stage to rolls)
Take the dough out of the fridge and leave to soften to room temperature for 10 minutes
Pre-heat oven to 160°C Fan/180°C/350°F. Line 2 baking tray with parchment paper.
Scoop two tablespoons each of the cookie batter and roll into balls. Place cookie balls on the baking tray at least 2 inches apart. If desired, press a few chocolate chunks on top of the cookie dough balls.
Bake for 10 – 12 minutes. Remove from oven and place on cooling rack. The centre of cookies may look slightly under baked. Leave to cool for at least 10 minutes before serving.