In a medium microwaveable mixing bowl, melt the butter in the microwave in 20-second intervals until melted. Whisk in the brown sugar, instant espresso, vanilla, baking powder, and salt. If the mixture is very hot, allow to cool slightly before adding the eggs. Whisk in the eggs, then add the flour and mix to combine. Lastly, add the chocolate chips. Transfer to the baking pan and bake for 22-26 minutes, until the top is shiny and crackly. Do not overbake. Allow to cool completely and cut into even squares.
