Preheat the oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper or lightly spray it with nonstick spray.
Whisk together the flour, cocoa powder, baking soda, baking powder, and sea salt in a small bowl; set aside.
In a large microwave-safe bowl, combine the butter pieces and the 3.5 ounces of chopped semisweet chocolate. Heat in 15-second intervals for 45–60 seconds total, stirring between intervals, until mostly melted. Stir until smooth and let cool 5 minutes.
Stir the granulated sugar into the cooled chocolate mixture. Add 2 large eggs and 1 large egg yolk and the vanilla; whisk until smooth.
Fold the dry ingredients into the wet mixture until almost combined, then fold in the chocolate chips until evenly distributed and no dry streaks remain.
Spread the batter evenly in the prepared pan and bake 25–30 minutes, or until the center is set and the top is puffed. Remove from the oven and let cool to room temperature (the center may sink slightly).
Make the ganache by placing the 9 ounces chopped milk chocolate in a heat-proof bowl. Heat the 1/2 cup heavy cream in a small saucepan until it just begins to simmer, then pour it over the chocolate. Let sit 30 seconds, then stir until smooth.
Pour the ganache over the cooled brownies, reserving about 1/4 cup if you want to drizzle extra later.
While the ganache is still wet, evenly sprinkle the peanut butter cups, kettle corn, and crushed potato chips on top, pressing lightly so they adhere.
Cover and refrigerate the brownies until the ganache is set, at least 2 hours or overnight. Slice and serve.