Preheat oven to 350°F. Line an 8x8 square pan with parchment paper and butter generously.
In a microwavable bowl, melt butter and ¾ cup chocolate chips until melted and smooth. Stir in 30 second intervals. Stir in sugar, vanilla and espresso.
In a small bowl, whisk the eggs until bubbly on top, about 1 minute. Add the eggs to the chocolate mixture.
In another small bowl, sift together the flour, cocoa powder, baking powder and salt. Add the dry ingredients to the wet ingredients and mix until just combined and then add the remaining chocolate chips.
Spread half of the batter evenly into the pan. Place 16 Kit Kat pieces on top and the spread the remaining batter over it. Crush the 4 remaining Kit Kat pieces and sprinkle on top.
Bake for 35-40 minutes. A cake tester should come up with a couple of crumbs. Let cool and then cut into 9 squares.
Notes
Grease and line your pan with parchment paper so you can easily get the brownies out.
Don’t overbeat the eggs as it introduces air and makes the brownies more cake-like and less dense. You should beat just until bubbles form on top.
Sifting together the dry ingredients fluffs them up and allows the wet ingredients to bind more quickly and evenly.
You will know when the brownies are done when you insert a toothpick and a few crumbs are on it. The toothpick should not come out clean, otherwise it is overbaked at that point.
Let the brownies cool completely before cutting them so you can get clean cuts.
You can double the recipe and bake in a 9x13 pan. It will probably take a few minutes longer to bake.