Preheat your oven to 350°F (175°C). This ensures that your brownies bake evenly.
Grease your 9x13-inch baking pan with butter or line it with parchment paper for easy removal later.
In a medium saucepan, melt 1 cup of butter and 6 ounces of chopped dark chocolate over low heat, stirring constantly until smooth. Remove from heat and let it cool slightly.
In a large mixing bowl, combine the melted chocolate mixture with 1 cup of granulated sugar and 1 cup of packed brown sugar. Mix until well combined.
Add 3 large eggs and 1 teaspoon of vanilla extract to the sugar mixture, whisking until smooth.
Gradually add 1 cup of all-purpose flour to the mixture, stirring until just combined. Be careful not to overmix.
Gently fold in 1 (12-ounce) bag of chopped mini Reese's peanut butter cups, reserving a handful for topping later.
In a separate bowl, using an electric mixer, beat 1 (8-ounce) package of softened cream cheese and 1/2 cup of softened butter until creamy. Add 1 egg, 1/2 cup of granulated sugar, 1/4 teaspoon of salt, 1 teaspoon of vanilla extract, and 1 cup of creamy peanut butter. Mix until fully combined and smooth.
Pour half of the brownie batter into the prepared baking pan, spreading it evenly. Next, carefully dollop the cheesecake mixture over the brownie base. Spread it gently to ensure even coverage, then pour the remaining brownie batter on top, spreading it to cover the cheesecake.
Sprinkle the reserved chopped Reese's peanut butter cups and 1 (12-ounce) bag of milk chocolate chips over the top of the brownie batter.
Bake in the preheated oven for 30-35 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Be careful not to overbake; you want that fudgy texture!
Allow the brownies to cool in the pan for at least 30 minutes before slicing. For the best texture, refrigerate for an hour before serving. Cut into squares and enjoy your Reeses Cheesecake Brownies!