01 - Combine gluten-free flour, sugar, salt, and instant yeast in a large bowl. Whisk warm milk and egg together, then add to dry ingredients. Mix until soft dough forms, then beat in softened butter until fully incorporated. Cover and refrigerate for 1 hour.
02 - Place cold butter between two sheets of parchment paper. Pound and roll into a 5 x 7-inch rectangle. Refrigerate until firm.
03 - Roll chilled dough on lightly floured surface into a 10 x 14-inch rectangle. Place butter layer in center and fold dough over to encase completely. Roll gently to 10 x 14-inch rectangle, then fold into thirds like a letter. Wrap and chill for 30 minutes. Repeat rolling, folding, and chilling process two more times for three complete folds.
04 - Roll dough into a 12 x 16-inch rectangle. Cut into 8 triangles. Starting from the wide end of each triangle, roll tightly into crescent shape. Arrange on parchment-lined baking sheet with pointed tips tucked under.
05 - Cover loosely and let rise in warm place for approximately 2 hours until puffy and doubled in size. Preheat oven to 400°F. Whisk egg yolk with milk and brush generously over risen croissants. Bake for 18-20 minutes until deep golden brown.
06 - Transfer croissants to wire rack and cool for at least 15 minutes before serving to allow interior structure to set.