In a medium sauce pan, melt the butter over medium high heat. Add the sugar, cocoa powder and espresso powder once the butter is melted. Whisk to combine and remove from heat.
Add the salt, vanilla and eggs and continuously whisk until the eggs are combined. Add the flour and continue to mix.
Transfer the batter into a 9x9 parchment lined baking pan. Top with the chocolate chips. Transfer to the oven and bake for 25-30 minutes until done. Remove and let cool for 2 hours before slicing and serving.
Notes
I love to line my pans with an overhang when making brownies. This allows you to lift the entire brownies out (once totally cool!) for the easiest slicing.