Preheat oven to 350℉.
(optional but recommended) Make brown butter by melting unsalted butter over medium heat. Stir slowly once melted and increase speed as the butter heats up and boils. Once you see brown bits at the bottom, remove from heat and transfer to a heat-safe container (could be the mixing bowl you use for the next step).
(optional) If you need to create your own dark brown sugar, combine 2 cups (400g) of granulated white sugar and 4 tablespoons of molasses.
In the mixing bowl of a stand mixer, add the butter (browned or otherwise) and dark brown sugar. Mix using the paddle attachment until well combined. The mixture should be a bit wet after mixing.
Add espresso powder, vanilla extract, and eggs. Mix until well combined.
Add the all-purpose flour and mix until a dough is made. Then add the milk chocolate chips and dark chocolate chips. Also add the walnuts (or your alternative of choice)
Tip out the dough onto a 9x13 baking pan lined with parchment paper. Spread with a spatula.
Bake in the oven preheated to 350℉ for 30 to 32 minutes or until the top is shiny and golden. A cake test will not come out clean.
Remove from oven and let cool. These can be cooled overnight if needed.
