Millionaire's Shortbread Recipe - Once Upon a Chef

Ingredients

• 1 cup all-purpose flour, spooned into measuring cup and leveled-off
• ¼ cup dark brown sugar
• 2 teaspoons cornstarch
• ½ teaspoon salt
• 1 stick (½ cup) cold unsalted butter, cut into ½-inch cubes
• 1 large egg yolk
• 1 tablespoon ice water
• 1 stick (½ cup) unsalted butter
• 1 (14-ounce) can sweetened condensed milk
• ½ cup heavy cream
• ½ cup light corn syrup
• 1 teaspoon vanilla extract
• 1 cup (packed) dark brown sugar
• 3 ounces semi-sweet chocolate, chopped
• 3 ounces milk chocolate, chopped
• Flaky sea salt, for serving