Preheat the oven to 350°F (175°C). Line an 8×8 inch baking pan with parchment paper, leaving an overhang on the sides for easy removal.
In a medium bowl, whisk together the gluten-free all-purpose flour blend, baking soda, and salt. Set aside.
In a large bowl, combine the melted unsalted butter, light brown sugar, and granulated sugar. Whisk until smooth and well combined.
Add the egg and pure vanilla extract to the butter and sugar mixture. Whisk until fully incorporated.
Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Do not overmix.
Fold in the semi-sweet chocolate chips evenly throughout the dough.
Spread the dough evenly into the prepared baking pan, pressing gently to create an even layer.
Bake for 22 to 25 minutes, or until the edges are golden and a toothpick inserted into the center comes out with a few moist crumbs.
Remove from the oven and allow the cookie bars to cool completely in the pan on a wire rack.
Once cooled, use the parchment paper overhang to lift the bars out of the pan and transfer to a cutting board. Cut into 16 equal bars and serve.