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 until well combined.
In a large bowl, mix the melted butter, light brown sugar, and granulated sugar until smooth and creamy.
Add the egg and vanilla extract to the butter and sugar mixture and whisk until fully incorporated.
Gradually add the dry ingredients to the wet ingredients, stirring until just combined. Do not overmix.
Fold in the semisweet chocolate chips evenly throughout the batter.
Spread the batter evenly into the prepared baking pan, smoothing the top with a spatula.
Bake in the preheated oven for 22 to 25 minutes, or until the edges are golden brown and a toothpick inserted in the center comes out with a few moist crumbs.
Remove the pan from the oven and allow the bar cookies 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. Cut into 16 squares and serve.