Coat a 9-by-13-inch baking dish with cooking spray. Line with parchment, leaving a 2-inch overhang on the 2 long sides.
Combine wafers, oats, sugar, and salt in a large bowl. Melt butter in a medium saucepan over medium heat, then add chunky peanut butter and 3/4 cup smooth peanut butter, whisking until well combined. Add peanut butter mixture to wafer 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.
Heat remaining 3 tablespoons smooth peanut butter in a small saucepan until runny. Drizzle peanut butter over chilled chocolate. Drizzle melted milk chocolate over peanut butter. Refrigerate until hardened, about 15 minutes.
Use parchment to lift out chilled block of bars. Run a sharp knife under hot water, dry well, and cut into 24 squares, wiping knife between cuts. Let bars stand at room temperature for 10 minutes before serving. (Sliced bars can be refrigerated for up to 1 week or frozen for up to 1 month.)