Pre-heat the oven to 350º F, and line a 9x13 baking pan with parchment paper.
In a large mixing bowl, mix together the melted butter, brown sugars, and sugar.
Add the eggs and vanilla extract, mixing well.
In a separate mixing bowl, whisk together the flour, xanthan gum, baking powder, baking soda, and salt.
Mix the dry ingredients into the creamed mixture.
Add the white chocolate chips and walnuts, thoroughly mixing them both into the dough.
Spread the dough in the parchment paper lined baking dish.
Bake at 350º F for 45 minutes, or 'til a toothpick inserted into the center comes out mostly clean with just a few crumbs.
Allow the blondies to cool; then cut into squares, remove them from the pan, and serve.