First, let's make brown butter; melt unsalted butter in a saucepan over medium heat. Once melted, continue cooking until the butter turns golden brown with a nutty aroma. Remove from the heat and let cool.
Preheat the oven to 350°F/177°C and line two baking sheets with parchment paper. In a small bowl, dissolve the coffee granules with a tablespoon of hot water.
In a large bowl, mix melted butter and the sugars until well combined.
Add room-temperature eggs one by one, beating well after each addition. Stir in the vanilla and espresso powder.
In a medium bowl, mix flour, baking powder, baking soda and salt.
Gradually add the flour mixture to the wet ingredients, mixing until well combined.
Fold in the chocolate chips using a rubber spatula until evenly distributed in the cookie dough.
Move the dough to a large baking sheet, cover it with plastic wrap, and let it rest in the fridge overnight.
Next day, scoop cookie dough using a cookie scoop and place them on the prepared baking sheet, sprinkling with flaky sea salt. Cover and put in the freezer for 30 minutes.
Bake in a preheated oven for 12-15 minutes or until the edges are golden brown but the center is still gooey. Baking time is indicative; It could take some extra minutes depending on your oven.
Move the baked cookies to a cooling rack.