Step 1Line a deep square baking tray with parchment, leaving a 2-inch overhang on the 2 long sides.
Step 2Combine biscuits, oats, sugar, and salt in a large bowl.
Step 3Melt butter in a medium saucepan over medium heat, then add chunky peanut butter and smooth peanut butter, whisking until well combined.
Step 4Add peanut butter mixture to dry mixture, stirring until combined and melted.
Step 5Transfer to tray, and use the bottom of a measuring cup or an offset spatula to firmly press mixture into an even layer.
Step 6Refrigerate for 30 minutes.
Step 7Melt chocolate in microwave and pour over chilled mixture and, using an offset spatula, spread into a thin layer that covers the surface
Step 8Refrigerate until hardened, at least 15 minutes.
Step 9Heat remaining 3 tbsp of smooth peanut butter in a small saucepan until runny.
Step 10Drizzle peanut butter over chilled chocolate.
Step 11Refrigerate until hardened, about 15 minutes.
Step 12Use parchment to lift out chilled block of bars.
Step 13Run a sharp knife under hot water, dry well, and cut into 24 squares, wiping knife between cuts.
Step 14Let bars stand at room temperature for 10 minutes before serving.