In large mixing bowl, stir together butter, sugar, egg yolk and vanilla until combined.
Add in flour, xanthan gum, and salt and stir until everything is combined and the mixture forms a dough.
Spread the dough on the bottom of an ungreased 9"x13" baking pan. Use your fingers to press the dough evenly around the pan and into the corners.
Bake for 25 minutes or until lightly browned.
While the dough is baking, break the candy bar into pieces. (Note: I use a popular milk chocolate bar that breaks up easily.)
After taking the pan out of the oven, lay the chocolate pieces all over the top of the bars. Wait 5 minutes while the chocolate softens, then spread evenly over the baked layer.
Sprinkle the walnuts over the top of the chocolate and let cool for 30 minutes.
Cut into bars while still warm.
Notes
You don't need to add xanthan gum, if the gluten free flour mix you use already contains it. Recipe adapted from Betty Crocker