Preheat your oven to 350°F (175°C).
Grease a 9x13-inch baking pan with butter or cooking spray, or line it with parchment paper.
In a large mixing bowl, cream together the softened butter and granulated sugar for about 3-4 minutes until light and fluffy. Add in the sour cream, vanilla extract, and the egg, mixing until well combined.
In a separate bowl, whisk together the gluten-free flour, baking powder, baking soda, and salt.
Gradually add the dry mixture to the wet mixture, stirring gently until just combined.
Pour the batter into the prepared baking dish and spread it evenly.
Bake in the preheated oven for 25-30 minutes, until the edges are lightly golden and a toothpick inserted in the center comes out clean.
Allow the cookie bars to cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely before frosting.