Preheat the oven to 350°F/180°C. Line a square 8×8 baking pan with parchment paper and set it aside.
Brown the butter: Melt the butter in a medium sized pan over medium heat. Once it melts, it will crackle, then become foamy – continue mixing the butter while this happens to prevent burning. The butter will go from melted to golden brown quickly, and you'll know it's almost done when the aroma of browned butter arrives. The whole process should take about 5 minutes! Once golden, remove from heat and transfer to a glass bowl.
1 cup butter
Whisk in the dark chocolate chips until melted with the brown butter, it'll be smooth and glossy.
1 2/3 cups dark chocolate chips
In a separate small bowl, beat eggs, vanilla, and sugar on medium speed until pale and fluffy. About 2 minutes.
1 cup white sugar, 3 eggs, 2 tbsp vanilla
Slowly beat into the chocolate mixture until combined.
Add in cocoa, flour, espresso powder, and salt. Fold until just mixed.
1 cup all-purpose flour, 4 tbsp dark cocoa powder, 2 tbsp espresso powder, 3/4 tsps salt
Transfer into prepared baking pan. Bake for 25-30 minutes. The top will no longer look shiny, and it'll be extremely soft. A toothpick inserted should come out with moist crumbs, not wet like batter.
Allow to cool completely in the pan before slicing. It'll fall apart if you cut into it earlier. Add a dash of sea salt on top to balance the flavours if desired. Enjoy!