• Preheat your oven to 325°F. Grease and line an 8-inch square baking pan with parchment paper, then set the pan 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 with xanthan gum, 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. Run it around the perimeter of the pan to create some separation.
• 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 briefly.

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. Mix to combine.
• Microwave again on high power for 5 minutes. Remove from the microwave and mix again.
• Microwave again on high power for another 2 to 4 minutes or until done. It's done when the mixture has begun to turn a bit more amber in color and 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 chocolate pieces in a medium-sized, heat-safe bowl.
• Bring the cream to a simmer over 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 or whisk until smooth.
• Pour the chocolate over the cooled caramel layer and spread it into an even 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 at room temperature or until no longer hot to the touch.
• Refrigerate until the bars are firm enough to hold their shape when lifted out of the pan (about 1 hour).
• Slice into 9 equal squares with a sharp knife, and serve chilled or at room temperature.
• If the caramel layer is hard to bite or slice after chilling, allow to sit at room temperature for 10 minutes before slicing or serving.