Preheat oven to 350F. Line an 8x8 pan with foil and spray with cooking spray.
Make the shortbread by creaming the butter and sugar with a hand or stand mixer until fluffy. Mix in vanilla and then stir in flour. Press into the bottom of the prepared pan. Bake for 16-18 minutes until slightly golden brown on top. Cool about 20 minutes before continuing.
Make the caramel layer by adding caramels, peanut butter, and heavy cream in a microwave safe bowl. (Before heating, have an offset spatula or an extra metal spoon and more cooking spray handy for spreading.) Heat for 40 seconds on HIGH, stir, then melt for an additional 10-20 seconds. Stir until smooth. Pour on top of shortbread. Use an offset spatula or metal spoon coated with cooking spray to evenly spread in the pan. Chill 10 minutes before adding chocolate.
Make the chocolate topping by melting chocolate chips and peanut butter together in the microwave, about 30-45 seconds. Stir until smooth and pour over caramel. Spread carefully. Top with Reese’s Pieces, if desired. Chill until completely firm, several hours.
Before slicing, let the bars warm up a little. The caramel gets hard, and it will be hard to cut until they start to come to room temperature. Best eaten at room temp, so the caramel doesn’t hurt your teeth!