• Preheat your oven to 325°F. Line an 8-inch square baking pan with parchment paper and set it aside.

Make the shortbread

• 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 blend, 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 use a small offset spatula or knife to spread it into an even layer.
• Bake in the center of the preheated oven 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.

Make the caramel

• 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.
• Microwave again on high power for 5 minutes. Remove from the microwave and stir.
• Microwave again on high power for another 3 to 4 minutes or until done. It's done when 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.
• Pour the hot caramel mixture into an even layer on top of the cooled shortbread.
• Bang the pan flat on the counter to break any large air bubbles. Allow to set at room temperature.

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 it over the chopped chocolate.
• Allow the mixture to sit for about 1 minute, then stir until smooth.
• Pour the chocolate over the cooled caramel layer. 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.

Chill and slice

• Allow the bars to chill for about 30 minutes in the refrigerator until they’re firm enough to hold their shape when lifted out of the pan.
• Slice into 12 equal squares with a sharp knife, and serve chilled.
• If the caramel layer is hard to bite or slice, allow to sit at room temperature for 10 minutes before slicing or serving.