Preheat your oven to 350℉ and gather your ingredients. Measure out 1 lb. (16 oz) of chocolate chips and set the remaining 10 oz aside.
Set up a 'Bain Marie' by placing a small pot filled ⅓ of the way with water to a full boil, ensuring that your metal bowl can be placed over the pot without the bottom of the bowl directly touching the water.
While the water heats up, add 16 oz. of chocolate chips, the chopped, unsweetened chocolate and the butter to the large metal bowl. Reduce the heat, then place the bowl over the simmering water.
Using a rubber spoon, stir the chocolate mixture fairly frequently, until the consistency is smooth and drips freely like a ribbon when the spoon is held over the bowl. Remove the bowl from the heat and set aside to cool slightly.
In the meantime, crack your eggs into a large mixing bowl. Gently mix in the sugar, espresso powder and vanilla.
Using a fine mesh strainer, sift together the flour, baking powder, and cinnamon. Whisk in the kosher salt.
Once the chocolate mixture has cooled, slowly add into the egg mixture, folding the ingredients together until smooth.
A little at a time, fold the flour mixture into the batter. Be sure there are no streaks of flour.
Pour the brownie batter into the prepared pan, and sprinkle evenly with the reserved chocolate chips. Use the spatula to work the chips into the mixture so they are not just laying on top.
Bake the brownies in the preheated oven for approximately 25 minutes, or until a toothpick inserted into the center comes out clean.
Once the brownies are baked, remove from the oven and allow the pan to cool to room temperature, before placing them in the refrigerator to fully set. Note: Brownies are SUPER fudgy, so cutting them before they are fully set is not advised. Cut fully chilled brownies into five rows vertically, four rows horizontally, to create 20 even pieces.
Dust with powdered sugar before serving, if desired.