Combine flour, whey protein, xanthan gum, baking powder, and salt in a medium bowl. Cut chilled butter into flour mixture with pastry cutter or with your hands, until butter is the size of a pea or smaller. Stir in sour cream. Mixture will still seem dry.
Using hands, squeeze and knead mixture to become a cohesive dough. Pour out onto a surface once dough beings to come together and knead a few times until dough is just smooth.
Roll out dough on a lightly floured surface to a 4- x 5-inch rectangle. Fold dough into thirds, as folding a letter.
Roll dough again into a 4- x 5-inch rectangle. Fold dough into thirds, as folding a letter. Wrap dough in plastic wrap and chill for at least 30 minutes.
Preheat oven to 400°.
Roll out dough on a lightly floured surface to a 10- x 10-inch square. Trim edges with a knife or pastry wheel to make a clean square. Cut square into 4 (5- x 5-inch) squares.
Working with one square at a time, make two parallel lines of chocolate chips down the center of the square. Fold parallel edges of pastry over chocolate, folding slightly under chocolate, and pressing pastry edges to close seam gently with fingertips.
Flip pastry over and place, seam side down, on a baking sheet lined with parchment.
Repeat with remaining pastry squares and chocolate.
Bake at 400° for 18-20 minutes or until pastries are puffed and golden. Allow to cool slightly before eating. Best served slightly warm. Heat leftovers (if you've got them) gently in a warm oven or toaster oven.