Whisk together the gluten free flour blend, baking powder, and fine sea salt in a medium bowl. Set aside.
In a large bowl, beat the softened butter and granulated sugar with an electric mixer on medium-high speed until light and fluffy, about 3 minutes. Add the eggs one at a time, beating well after each. Mix in the ube extract and vanilla extract until the dough is a vibrant purple color.
Add the flour mixture and mix on low speed until just combined into a soft dough. Fold in the white chocolate chips if using. Cover the bowl tightly and refrigerate for at least 2 hours or overnight until the dough is very firm.
Preheat the oven to 350 degrees F and line two baking sheets with parchment paper. Place the powdered sugar in a shallow bowl.
Scoop the chilled dough into 1-tablespoon balls and roll each one generously in the powdered sugar until thickly coated. Place them about 2 inches apart on the prepared baking sheets.
Bake for 11 to 13 minutes until the tops are puffed and beautifully crackled with vibrant purple peeking through the white powdered sugar. The centers should still feel soft.
Let the cookies cool on the baking sheet for 5 minutes before transferring to a wire rack. The cookies will be soft and chewy inside with a stunning crinkle exterior that looks absolutely gorgeous.