First, make your cookie dough! In a large bowl, using an electric mixer, cream together the butter, brown sugar, and white sugar until lighter and fluffy (3-5 minutes – don't skimp on this step!). Add in the egg plus egg yolk and ube extract and beat until smooth, another 30 seconds. Add in the flour, xanthan gum, cornstarch, baking soda, and salt and mix until fully combined (it will be fairly stiff). Finally, add in the chopped white chocolate and stir to combine with a spatula or wooden spoon. Scoop dough into a smaller bowl, cover tightly with plastic wrap, and chill in the fridge for 1 hour.