Coat an 8x8-inch baking dish with cooking spray. Line with parchment, leaving a 2-inch overhang on the 2 long sides.
Combine graham cracker crumbs, oats, sugar, and salt in a large bowl.
1 cup chocolate graham cracker crumbs, ¾ cup rolled oats, ¾ cup powdered sugar, Pinch kosher salt
Melt butter in a medium saucepan over medium heat, then add chunky peanut butter, smooth peanut butter, and vanilla, whisking until well combined.
5 tablespoons butter, ½ cup chunky peanut butter, 6 tablespoons creamy peanut butter, ½ teaspoon vanilla extract
Add peanut butter mixture to crumb mixture, stirring until combined.
Transfer to baking dish, and use the bottom of a measuring cup or an offset spatula to firmly press mixture into an even layer. Refrigerate for 30 minutes.
Pour melted semisweet chocolate over chilled mixture and, using an offset spatula, spread into a thin layer that covers the entire surface. Refrigerate until hardened, at least 15 minutes.
5 ounces semisweet chocolate, melted
Heat remaining smooth peanut butter in a small saucepan until runny.
1 tablespoon creamy peanut butter
Drizzle peanut butter over chilled chocolate. Drizzle melted milk chocolate over peanut butter. Refrigerate until hardened, about 15 minutes.
1 ounce milk chocolate
Use parchment to lift out chilled block of bars. Using a sharp knife, cut into 12 squares. Let bars stand at room temperature for 10 minutes before serving.
Notes
Graham crackers: If you can't find chocolate graham crackers, you can use honey grahams. The bars just won't have quite as much chocolate flavor.
Peanut butter: I don't recommend using natural peanut butter because it can be oily, which doesn't work well in this recipe.
Chocolate: You can use all milk chocolate or all semisweet chocolate if you prefer. Using milk chocolate will make the bars a bit sweeter.