For the shortbread layer:
2 cups all-purpose flour
2 tsp. baking powder
¼ tsp. salt
1 cup (2 sticks) unsalted butter, at room temperature
½ cup sugar
For the caramel layer:
1 cup (2 sticks) unsalted butter, cut into pieces
1 cup sugar
4 tbsp. light corn syrup
2 (14 oz.) cans sweetened condensed milk
For the chocolate layer:
8 oz. semisweet or bittersweet chocolate, finely chopped
1 tsp. light corn syrup
½ cup (1 stick) unsalted butter, cut into pieces
Fleur de sel or sea salt, for sprinkling