For the shortbread layer
8 tablespoons (112 g) unsalted butter, at room temperature
1/3 cup (73 g) packed light brown sugar
1/8 teaspoon kosher salt
1 cup (140 g) all purpose gluten free flour (I used Better Batter)
1/4 teaspoon xanthan gum (omit if your blend already contains it)
1/4 cup (36 g) cornstarch
For the caramel layer
8 tablespoons (112 g) unsalted butter, chopped
1 1/4 cups (273 g) packed light brown sugar
1/2 cup (168 g) light corn syrup
7 ounces (1/2 can) sweetened condensed milk
For the chocolate layer
12 ounces bittersweet, dark or milk chocolate, chopped
3/4 cup (6 fluid ounces) heavy whipping cream
Coarse salt, for finishing (optional)
Preheat your oven to 325°F. Line an 8-inch square baking pan with parchment paper and set it aside.
To make the shortbread layer, in a large bowl, place the butter, brown sugar, and salt. Beat with a handheld mixer or stand mixer fitted with the paddle attachment on medium-high speed until light and fluffy (about 3 minutes). Add the flour, xanthan gum and cornstarch, and mix until just combined. The dough will seem crumbly at first, but just keep mixing until it comes together. Transfer the cookie dough to the prepared baking pan, and press it into an even layer. Place the pan in the center of the preheated oven and bake until lightly golden brown on the edges and set all the way to the center (about 18 minutes). It’s set in the center when it no longer shines. Remove the pan from the oven and set it aside to cool completely.
While the shortbread is cooling, making the caramel layer. Place all of the caramel ingredients in a large microwave-safe bowl. Use the largest bowl that will fit into your microwave since the mixture will bubble as it cooks. Place the bowl in the microwave and cook on high power for 1 minute or until the butter is completely melted. Remove the bowl from the microwave and mix to combine. Return the bowl to the microwave and cook on high power for 5 minutes. Remove from the microwave and stir. Return the bowl to the microwave and cook again until the mixture has begun to darken slightly in color and has either reached about 235°F using an instant-read thermometer or can be pressed into a soft ball when dropped in a bowl of cold water. That should take another 3 to 4 minutes. The mixture will have changed in color to a slightly darker golden brown. The bowl will be quite hot by now. Pour the hot caramel mixture over the cooled shortbread, bang the pan flat on the counter to break any large air bubbles, and allow to set at room temperature.
Finally, make the chocolate layer. Place the chopped chocolate in a medium-sized, heat-safe bowl. Bring the cream to a simmer over medium-low heat on the stovetop (or in the microwave) and pour over the chopped chocolate. Allow the mixture to sit for about 1 minute before stirring until smooth. Pour the chocolate over the cooled caramel layer, and bang the pan flat on the counter a few times to break any large air bubbles. Allow to sit for about 5 minutes before sprinkling with the optional coarse salt.
Allow the bars to chill for about 30 minutes in the refrigerator until they’re firm enough to hold their shape when they’re lifted out of the pan. Using a sharp knife, slice into bars and serve chilled. If you allow the bars to chill so much that the caramel layer is hard to bite or slice, allow to sit at room temperature for 10 minutes before slicing or serving.