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 sift flour, baking soda, cornstarch and salt in a large mixing bowl. Set aside.
In the bowl of a stand mixer, beat together brown butter, brown sugar and white sugar . Whisk the eggs and vanilla extract until light and creamy. Pour the egg mixture into the sugar mixture. Mix in the flour mixture .
Fold in the chocolate chips and chunks.
Scoop two tablespoons each of the cookie batter and roll into balls. Cover the cookie balls with plastic wrap and leave to chill in the fridge for at least 2 hours or overnight.
Line 2 baking tray with parchment paper. Remove the cookie dough and leave to come to room temperature. Place cookie balls on the baking tray at least 2 inches apart.
Remove the cookie dough and leave to come to room temperature for 10 minutes.
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.