Preheat oven to 375°. Generously butter (or spray) an 8-inch square baking pan. Line the bottom and sides of the baking pan with parchment paper, leaving 3 inches of overhang on all sides, and lightly spray.

For the crust: Combine flour, sugar, cornstarch, and butter in a food processor. Pulse until mixture resembles sand. Press the dough into the baking pan, making sure to press evenly and into each corner. Bake for 20 minutes. Remove from oven and allow to cool completely.

For the caramel filling: In a small saucepan or skillet, combine the sugar, water, and corn syrup, and bring to a boil over medium-high heat. Do not stir. When the sugar mixture turns a pale to medium amber color, reduce the heat and carefully add the cream. Whisk in butter and salt. Pour the hot caramel over the shortbread and set aside.

For the chocolate topping: In a saucepan, add 1 cup of water. Place a glass bowl over the saucepan to create a double boiler. Add the chocolate and the butter to the bowl and melt over low heat, stirring occasionally. Do not let the water boil. When the chocolate has melted completely, pour over the caramel layer and let sit for 10 minutes. Sprinkle with flaked sea salt.

Using the extra parchment paper, lift the dessert out of the pan and cut into bars, cleaning the knife between each slice. Serve at room temperature or chilled.