Preheat oven to 325 degrees F.

Prepare an 8-inch square baking pan by lining the pan with aluminum foil, leaving 3-4 inches of foil draping over the sides of the pan.

The crust:

Place graham crackers into the bowl of a food processor and pulse until the crackers form fine crumbs, about 30 seconds. To the crumbs, add butter, brown sugar, flour, and salt. Pulse until the ingredients are fully combined. Using the bottom of a measuring cup or your fingers, press the crumbs into the bottom of the prepared pan.

Bake until the crust is beginning to brown, about 12 to 15 minutes. Let the crust cool on a cooling rack for about 30 minutes.

The filling:

In a large bowl, beat the cream cheese until it is very smooth, about 3 minutes. Scrape the beaters and sides of bowl frequently. Gradually beat in granulated sugar until the mixture is combined, about 1 minute. Beat in eggs, one a time, making sure that each is fully incorporated. Beat in sour cream and vanilla extract until fully combined, about 30 seconds.

Unwrap and cut 9 Reese’s peanut butter cups (0.75 ounce each) into 6 pieces. Gently fold into the cream cheese mixture. Scrape the cream cheese mixture into the baking pan, over the top of the cooled crust. Spread evenly.

Bake until the edges are set but the middle still jiggles slightly when the pan is shaken slightly, about 35 to 40 minutes, rotating the pan halfway through baking.

Place the pan on a cooling rack and allow the bars to cool for 2 hours. Cover with aluminum foil and refrigerate for at least 3 hours, and up to 24 hours.

Using the overhanging foil, lift the bars from the pan. Gently remove the foil and cut into 16 bars.

Place bittersweet chocolate in a small bowl. Microwave on 50% power until melted, stirring every 15 seconds. Drizzle the chocolate over the bars, top each with a small chunk of a Reese’s peanut butter cup and serve.