Preheat oven to 350 degrees and line a 9x13 baking pan with parchment paper; set aside.
First, measure the dry ingredients and whisk together in a small bowl; flour, cocoa powder, salt and espresso powder; set aside.
In another larger bowl, add eggs, vanilla, melted brown butter, sugar and brown sugar. Whisk vigorously until smooth.
To make the brown butter, place butter in a small/medium sauce pan and place over medium-low heat. Bring to a slight simmer and start stirring constantly until the butter has a nutty aroma and little brown bits start to form at the bottom of the pan. Immediately remove and pour into a bowl to cool down for 5 minutes. This is optional. If you don't want to brown the butter, simply melt the butter and let cool.
Next, add the bowl of dry ingredients to the bowl of wet ingredients and mix with a spatula. Lastly, add the chocolate chips and gently stir. The batter will be thick! Pour into your prepared pan and smooth out with a spatula.
Bake for 25-30 minutes. You don't want to overbake these, so error on less baking time if you're oven is a hot one. Let cool completely before removing from the pan using the edges of the parchment paper. Cut and enjoy!
