Preheat your oven to 350°F (175°C).
Line your 9x9 inch baking pan with parchment paper, allowing some overhang on the sides.
In a medium saucepan over low heat, melt the unsalted butter. Once melted, remove from heat and let it cool slightly.
In a large mixing bowl, combine the light brown sugar and granulated sugar. Pour the melted butter over the sugars and whisk until smooth.
Add the large egg, egg yolk, and vanilla extract to the sugar mixture. Whisk until fully incorporated and the mixture is glossy.
In a separate bowl, whisk together the gluten free flour, cornstarch, baking powder, and salt.
Gradually add the dry mixture to the wet mixture, stirring until just combined. Be careful not to overmix!
Gently fold in the white chocolate chips and toasted walnuts until evenly distributed throughout the batter.
Pour the blondie batter into the prepared baking pan, smoothing the top with a spatula. Bake for 25-30 minutes, or until the edges are golden brown and a toothpick inserted in the center comes out with a few moist crumbs.
Once done, remove the blondies from the oven and let them cool completely in the pan on a wire rack. Once cooled, lift them out using the parchment overhang and cut into squares.
