Preheat oven to 400°F (200°C) and line a baking sheet with parchment paper.
In a large bowl, whisk together gluten-free flour, sugar, baking powder, baking soda, salt, and xanthan gum.
Cut cold butter into the flour mixture using a pastry cutter or fork until the mixture resembles coarse crumbs with pea-sized butter pieces.
Gently fold in blueberries, coating them lightly with flour.
In a separate bowl, whisk egg, egg yolk, cream, vanilla extract, and lemon zest.
Pour wet ingredients into dry ingredients and gently mix until a soft, slightly sticky dough forms.
Transfer dough to a lightly floured surface and pat into a 1-inch-thick round.
Cut into 8 wedges and place on prepared baking sheet.
Brush tops with egg white and sprinkle with coarse sugar if desired.
Bake for 18–22 minutes, or until golden brown and cooked through.
Cool slightly on a wire rack before serving.