2 to 3tablespoonsvegetable shorteningthis keeps the chocolate from cracking when you slice it
Maldon saltor other flaky sea salt for sprinkling on top
Preheat oven to 350F. Line an 8-inch square baking pan with aluminum foil leaving a few inches of overhang. Spritz lightly with cooking spray; set aside.
Shortbread Crust - Combine all ingredients in a medium mixing bowl, and mix lightly with a spoon or your fingers just until until a sandy, crumbly mixture forms (do not over mix of the shortbread will be tough). Tip the mixture into the prepared pan and press crumbs down to form an even flat layer of crust with your fingers or the bottom of a flat measuring cup. Poke crust with a fork in a dozen or so places so steam can escape while it bakes. Bake for 12 to 15 minutes, or until crust has just barely set. It should not take on any color, but crusted over and set. While crust bakes, make the filling.
Peanut Butter Filling - Melt the butter in a small microwave-safe bowl, about 1 minute. In a bowl of a stand mixer (or using a hand mixer), add the peanut butter, sugar, melted butter and vanilla and mix until smooth and fluffy (2-3 minutes). Using an offset spatula or a butter knife, spread evenly over the (mostly) cooled crust. Cover lightly with foil and chill in the fridge for 15 minutes.
Caramel Layer - Drizzle caramel sauce evenly over the peanut butter layer (make sure the sauce is warm enough to be pourable but not hot). Pour it all over and spread it into corners with a knife. Cover again and chill for just a few minutes while you melt the chocolate.
Chocolate Layer - Combine the ingredients in a medium microwave-safe bowl and heat in 15-seconds bursts, stirring each time, until mixture is almost completely melted. Once it’s very close, give it a second and let the residual heat finish the melting. Remove pan from fridge and pour chocolate over caramel layer. Tilt the pan around to even the chocolate layer or, if needed, smooth it very, very lightly with an offset spatula or knife. Sprinkle salt over the top, to taste.
Cover pan tightly with aluminum foil, and place pan in the refrigerator for at least 3 hours, or overnight, before lifting out with foil overhang, and slicing and serving. We found freezing the bars for 30 minutes helped with cutting but let the bars sit at room temperature before serving.
If you buy jarred caramel sauce, make sure it's thick (i.e. it doesn't move too much when you turn the jar upside down). That way, the caramel layer won't spill out when you cut into the chocolate.