Preheat oven to 350 degrees.
In a large bowl, combine butter and brown sugar. Mix with an electric or handheld mixer on medium for 1 minute.
Add eggs, vanilla and whipping cream. Continue to mix for an additional minute.
In a separate, large bowl, combine flour, starch, xanthan gum, powder, soda and salt. Toss to combine.
Slowly add dry ingredients to butter/sugar mixture, mixing well after each addition.
Once dough is thoroughly combined, add white chocolate chips and walnuts. Mix in by hand.
Line a 9 x 13 inch baking dish with parchment paper and spray with non-stick cooking spray.
Spread dough out evenly in baking dish. Bake for 30-35 minutes, or until toothpick inserted in center comes out clean.