Preheat oven to 350 degrees F (190 degrees C). Place butter cubes in the freezer for 15 minutes.
Whisk together flour, xanthan gum, sugar, salt, and baking powder in a large bowl.
Cut in frozen butter using a pastry cutter until the butter pieces are about the size of peas. Mix in egg yolk and continue cutting in until thoroughly combined. Drizzle in ice water and stir to combine. The dough should just come together when pinched between your fingers.
Pour about 3/4 of the crumb mixture into an ungreased 9x9-inch baking dish. Press the mixture down firmly using the back of a spoon. Spread berries in one layer and sprinkle with remaining crumbly dough.
Bake in the preheated oven until the top is golden and sides are crisp and browned, 30 to 35 minutes. Cool completely before serving.