Make the shortbread crust: Combine flour, powdered sugar, and butter in a food processor until it resembles coarse crumbs. Press into the bottom of a 9-inch square baking pan.
Bake for 20-25 minutes, or until lightly golden.
Make the caramel filling: Combine sugar, heavy cream, and butter in a saucepan over medium heat. Bring to a boil, stirring constantly, and cook until a candy thermometer reads 245°F (118°C).
Remove from heat and stir in vanilla extract. Pour over the shortbread crust.
Make the chocolate ganache: Heat heavy cream in a saucepan until simmering. Pour over chopped chocolate and let sit for 1 minute. Stir until smooth.
Pour ganache over caramel filling. Refrigerate for at least 2 hours before serving.
Notes
For a richer flavor, use salted butter in the caramel filling. You can also add a sprinkle of sea salt on top of the ganache.