Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving some overhang for easy removal.
In a medium saucepan, combine the 12 oz of semi-sweet chocolate chips and 6 tbsp of unsalted butter. Heat over low heat, stirring frequently until completely melted and smooth. Remove from heat and let it cool slightly.
In a large mixing bowl, combine 3/4 cup of peanut butter and 1/2 cup of granulated sugar. Beat in 1 egg until well combined.
Once the chocolate mixture has cooled, stir it into the peanut butter mixture until smooth.
Add in the additional 3/4 cup of granulated sugar, 1 tsp of vanilla extract, and 3 room temperature eggs, one at a time, mixing well after each addition.
In a separate bowl, sift together 1/4 cup unsweetened cocoa powder, 1/3 cup cornstarch, and 1/2 tsp of salt. Gradually fold this mixture into the wet ingredients until just combined.
Gently fold in 1 cup of mini semi-sweet chocolate chips and the chopped 10 oz bittersweet chocolate until evenly distributed.
Pour the brownie batter into the prepared baking pan, spreading it evenly with a spatula.
Bake in the preheated oven for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs (not wet batter).
Allow the brownies to cool in the pan for at least 20 minutes before using the parchment paper overhang to lift them out. Cut into squares and enjoy!
