Preheat oven to 350℉.
In a large mixing bowl, cream together butter and sugar. Scrape sides of bowl as needed. When you think you have it fully combined, scrape the sides again and continue beating the mixture for a few more minutes.
Add powdered sugar to the butter mixture. Start mixing on low so the powdered sugar doesn't fly out of the bowl. Gradually increase speed of mixer and beat until mixture is light in texture and almost white in color. (I use a stand mixer and let it run at the second highest setting while I prepare the next ingredients.)
In a microwave safe bowl, melt chocolate chips in microwave. Stir every 30 seconds in order to not burn the chocolate. Once fully melted, set aside to cool slightly.
In a medium mixing bowl, sift together flour, cocoa powder, espresso powder, and salt. Set aside. (Don't skip sifting. This prevents you from having clumps of cocoa powder in your brownies.)
Add eggs and vanilla to butter mixture and begin mixing at low setting.
While continuing to mix butter and eggs, add melted chocolate. Beat on medium speed until fully combined. Scrape sides of bowl as needed.
With a sturdy wooden spoon or spatula, gradually stir dry ingredients into mixture.
Spread brownie batter into greased 9x13 baking pan. Bake 30 minutes or until test toothpick come out almost clean (you want to see a few bits of slightly undercooked batter on the toothpick).