I love transforming a few simple components into one home-run dessert. Using store-bought candies and a few extra ingredients takes the guesswork out of making caramel, and a hint of coffee gives my millionaire’s bars even more depth. These are guaranteed to give you all that you’d want in a gourmet bar cookie.
Preheat the oven to 350 degrees F. Grease an 8-inch-square baking pan and line it with parchment paper so that the parchment extends up all sides of the pan.
To make the shortbread layer: In the bowl of a stand mixer fitted with the paddle, cream the butter, sugar and salt together on medium speed until a grainy paste forms, 1 to 2 minutes. Scrape the bowl and paddle. Add the egg yolk, vanilla and 1 teaspoon water and blend on medium until smooth, about 1 minute. Add the flour and mix on low until a crumbly dough forms; it will look dry and that’s okay.
Transfer the dough to the prepared pan, breaking up any large pieces to evenly scatter the dough over the bottom. Use your hand to press the dough as evenly as possible into the pan. If the dough sticks to your fingers, moisten your hand with cool water. Use a fork to prick the dough all over the surface.
Bake the shortbread until firm to the touch and golden brown (the edges may be a bit darker than the center), 30 to 35 minutes. Cool the pan on a rack until just warm before topping with the caramel layer, about 30 minutes.
To make the caramel layer: Stir the cream, vanilla, coffee and salt together in a medium pot, preferably nonstick. Add the butter and caramels and melt over low heat, stirring continuously with a rubber spatula, until smooth, about 5 minutes; reduce the heat if the caramel begins bubbling. Immediately pour the caramel over the warm shortbread and smooth the top. The caramel will smooth and level as it cools. Allow to cool for about 2 hours before adding the final ganache layer.
To make the chocolate layer: Combine the chocolate chips, cream and instant coffee if using in a small microwave-safe bowl and microwave for 15 seconds. Stir about 5 seconds and continue heating in 15-second increments until smooth, 30 to 45 seconds total. Don’t overcook the mixture; if small bits of chocolate remain just gently stir until they melt.
Pour the chocolate ganache over the cooled caramel layer and smooth to the edges. Allow to set until firm, about 1 hour at room temperature or 30 minutes refrigerated.
Use the overhanging parchment to lift the bars out. Slice into 16 squares and lightly sprinkle with flaky salt if using. The bars will keep in an airtight container at room temperature for up to 3 days.