Preheat the oven to 350°F. Line an 8x8-inch square baking dish with parchment paper and spray with cooking spray.

Make the shortbread: Pulse the flour, butter, and powdered sugar in a food processor or cut together in a bowl with a pastry blender until the mixture resembles moist sand. Firmly press the mixture into the bottom of the prepared pan to form an even layer.

Bake: Bake until the shortbread appears slightly glossy with a toasted color, 20 to 30 minutes. Cool the shortbread layer on the countertop, at least 30 minutes, or in the refrigerator.

Make the caramel: Once the shortbread has cooled, make the caramel layer. In a medium saucepan over medium-low heat, combine the sweetened condensed milk, corn syrup, and butter. Constantly whisk until the mixture reaches a temperature of 220 to 225°F on an instant-read thermometer, always cooking over medium-low heat, about 15 minutes. It should be light golden brown. Remove from heat and stir in the vanilla. Pour the caramel on top of the shortbread, using a rubber or offset spatula to smooth it into an even layer. Cool in the fridge until set, about 30 minutes.

Top with chocolate: Melt the chocolate in a double boiler or in 20-second bursts in the microwave, stirring often. Pour the melted chocolate over the caramel layer, and use an offset spatula to smooth before sprinkling with flaky salt. Chill the millionaire bars in the fridge until set, at least 30 minutes, before slicing with a large knife. Clean your knife between slices to ensure the most uniform bars. Store in an airtight container in the refrigerator for up to 1 week, or wrap and freeze individually wrapped for up to 6 months. Love the recipe? Leave us stars and a comment below!