Heat oven to 350 degrees F and line a baking sheet with parchment paper.
In a large mixing bowl, add melted butter, brown sugar, and granulated sugar. Whisk together until smooth.
1/2 cup + 2 tbsp butter, 2/3 cup brown sugar, 1/3 cup granulated sugar
Add egg and vanilla extract, and whisk until creamy.
1 large egg, 2 tsp vanilla extract
Add flour, cornstarch, baking soda, and sea salt, and stir until just combined and a soft dough is formed.
1¾ cup gluten free all-purpose flour, 1 tsp cornstarch, 1 tsp baking soda, 1/4 tsp sea salt
Scoop 2 tablespoons of dough, and drop on the baking sheet 2-3 inches apart.
Bake for 9-12 minutes until the edges begin to set and the centers look puffy and underbaked.
Remove from the oven and sprinkle with flaky sea salt, if using. Cool for 10 minutes then transfer to cooling rack to cool completely.
Optional: Flaky sea salt