Preheat your oven to 350°F (175°C) and grease or line your 9x13 inch baking pan with parchment paper for easy removal.
In a medium saucepan over low heat, combine the coarsely chopped unsweetened and semi-sweet chocolate with the unsalted butter. Stir gently until completely melted and smooth. Remove from heat and let cool slightly.
In a large mixing bowl, whisk together the sugar and eggs until well combined. Add the vanilla extract and mix until fully incorporated.
Gradually pour the melted chocolate mixture into the egg and sugar mixture, whisking continuously to combine.
In a separate bowl, whisk together the all-purpose flour, 2 tablespoons of cocoa powder, 1 teaspoon of cinnamon, ½ teaspoon cayenne pepper, and salt. Slowly add the dry ingredients to the wet mixture, stirring until just combined.
Gently fold in the mini chocolate chips, ensuring they are evenly distributed throughout the batter.
Pour the brownie batter into the prepared baking pan and spread it evenly. Bake in the preheated oven for about 25-30 minutes or until a toothpick inserted into the center comes out with a few moist crumbs.
Once baked, remove the brownies from the oven and let them cool in the pan on a wire rack. After cooling, dust with powdered sugar for a beautiful finish.