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. 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. Chill in refrigerator until firm.
03 - Roll chilled dough on lightly floured surface into 10 x 14-inch rectangle. Place butter layer in center and fold dough over to completely encase. Roll out 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 12 x 16-inch rectangle. Cut into 8 triangles. Starting at wide end, roll each triangle into croissant shape. Place on parchment-lined baking sheet.
05 - Cover loosely and let rise in warm place for approximately 2 hours until puffy. Preheat oven to 400°F. Whisk egg yolk and milk for glaze, then brush over croissants. Bake for 18-20 minutes until golden brown.
06 - Transfer to cooling rack and let cool completely before serving.