For the Shortbread Crust
1/2 cup unsalted butter, extremely soft (1 stick)
1 cup all-purpose flour
1/3 cup confectioners' sugar
1 tablespoon cornstarch
1 teaspoon vanilla extract
For the Peanut Butter Filling
2 tablespoons unsalted butter, melted
1 cup creamy peanut butter (like Jif or Skippy; not homemade or 'natural', it's too thin and runny)
1/2 cup confectioners' sugar
1/2 teaspoon vanilla extract
For the Caramel Layer
about 8 ounces (salted) caramel sauce (I used most of a 10-ounce jar of Trader Joe's Fleur de Sel Caramel Sauce, just eyeball it. Or make Caramel Sauce or make 5-Minute Caramel Sauce)
For the Chocolate Layer
1 1/2 to 1 3/4 cups semi-sweet chocolate chips (I used a 9 ounce bag)
2 to 3 tablespoons vegetable shortening, optional but recommended (for smoothness and so chocolate doesn't set up overly firm, causing it to crack upon slicing)