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 bowl and refrigerate for 1 hour.
02 - Place cold butter between two sheets of parchment paper. Pound and roll into rectangle approximately 5 x 7 inches. Transfer to 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 butter. Roll out gently to 10 x 14-inch rectangle. Fold dough into thirds like a letter. Wrap and refrigerate for 30 minutes.
04 - Repeat rolling and folding process two more times for three total folds. Refrigerate for 30 minutes between each fold. After final fold, wrap dough and refrigerate for 30 minutes.
05 - Roll dough into 12 x 16-inch rectangle. Cut into 8 triangles. Starting from wide end, roll each triangle into croissant shape. Place shaped croissants on parchment-lined baking sheet, leaving space between each.
06 - Cover loosely with plastic wrap or clean towel. Let rise in warm place for approximately 2 hours until puffy and doubled in size.
07 - Preheat oven to 400°F. Whisk egg yolk and milk together for egg wash. Brush evenly over croissants. Bake for 18 to 20 minutes until deeply golden brown.
08 - Transfer croissants to wire rack and cool completely before serving.