In a small saucepan over medium heat, melt 1/2 cup sugar from the caramel, stirring constantly until golden amber. Remove from heat and whisk in 1/4 cup butter (for caramel). Slowly pour in 1/2 cup heavy cream and 1/4 teaspoon salt, whisking until smooth. Cool for 10 minutes.
Preheat oven to 350°F (175°C). Grease and flour an 8×8-inch baking dish with gluten-free flour or line with parchment paper.
In a large heatproof bowl, melt 1 cup butter and 4 ounces unsweetened chocolate using a double boiler or microwave, stirring until fully combined.
Add 2 cups sugar, 1 teaspoon salt (for brownies), and 1 teaspoon vanilla extract to the chocolate-butter mixture. Whisk until blended.
Incorporate 4 large eggs one at a time, mixing thoroughly after each addition.
Fold in 1 cup gluten-free flour blend and 1/2 teaspoon xanthan gum (if using). Stir in the cooled salted caramel and mix until just combined.
Pour the batter into the prepared 8×8-inch baking dish. Sprinkle flaky salt on top.
Bake for 30–35 minutes until a toothpick inserted in the center has a few moist crumbs but no liquid batter. Cool completely before slicing into 16 squares for serving.