Brown the butter: Melt the butter in a medium-sized saucepan over medium heat. Continue to cook the butter, while whisking constantly, until it’s a pale golden brown and the particles suspended in it are reddish brown. This usually takes me about 10 minutes.
Remove the pan from heat and stir in the sugar. Then add the cocoa powder, sea salt, baking powder and espresso powder. Stir until the ingredients are thoroughly incorporated. Let the mixture cool for 5 minutes.