Mix the butter, cocoa powder, and sugar until you have a smooth, chocolate mixture. This will be thickkkkkk. Add this concoction to a large mixing bowl.
Next, add the eggs one at a time until thoroughly incorporated. At this point, the mixture should be silky. Add the vanilla and espresso powder to combine. Fold in the flour and chocolate chips.
Pour brownie batter into a 9x13, parchment lined baking pan and bake for about 30 minutes. Test it with a toothpick and make sure the toothpick comes away fairly clean after removing from the oven. The chocolate chips in the batter can make this a little tricky. You just don't want a super wet batter on the toothpick.
Remove the brownies from the oven and let them cool completely. While this happens, let's move on to the mocha frosting.
The Mocha Frosting
Add the butter to a large mixing bowl along with the heavy cream and vanilla. Using either a stand mixer or hand mixer, begin to cream these ingredients together. Then, SLOWLY add the powdered sugar and cocoa powder a little at a time.
Once the sugar and cocoa are incorporated, add a healthy pinch of kosher salt and mix to combine.
With an offset spatula, carefully smooth the frosting over the surface of the brownies. Top with a healthy sprinkle of flakey sea salt.