Preheat oven to 350°F (177°C). Line the bottom and sides of a 9×13-inch baking pan with parchment paper, leaving an overhang on the sides to lift the finished brownies out (makes cutting easier!). Set aside.

Melt the butter and semi-sweet chocolate together in a medium saucepan on medium heat, stirring constantly (about 5 minutes). Remove from heat and let cool to room temperature. This takes about 15 minutes. Use this 15 minutes to make the chocolate chip cookie layer! Whisk sugar into cooled chocolate/butter mixture until combined. Add in the eggs one at a time, whisking until smooth after each addition. Whisk in the vanilla extract, then use a silicone spatula or wooden spoon to fold in the flour and salt until combined.

In a large bowl using a handheld mixer or stand mixer fitted with paddle attachment, cream the butter and sugars together on medium speed until combined and creamy. Beat in the egg and vanilla extract until combined. Scrape down the sides and up the bottom of the bowl as needed. Add the flour, cornstarch, baking soda, and salt. Beat on low speed until combined, then beat in the chocolate chips. Dough will be thick and sticky. Using a silicone spatula sprayed with cooking spray (to reduce sticking), spread the cookie dough all over the prepared baking pan. The layer will be thin, but will puff up as it bakes. Top with peanut butter cups, then pour the brownie batter on top and smooth with a knife or rubber spatula.

Bake the bars for 40-45 minutes, or until the center no longer jiggles and a toothpick inserted in the middle comes out relatively clean (a few brownie crumbs are OK). During bake time, tent pan with foil if the edges look like they are cooking a lot faster than the center.

Allow the bars to cool completely in the pan. Once cool, lift the parchment paper out of the pan using the overhang on the sides, then cut into squares.

Cover and store leftover brownies at room temperature for up to 1 week.