01 - Combine gluten-free flour, sugar, salt, and instant yeast in a large mixing bowl. Whisk warm milk and egg together until blended, then pour into dry ingredients. Mix until soft dough forms, then 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 butter with rolling pin to flatten, then roll into rectangle approximately 5 x 7 inches. Refrigerate until firm but pliable, about 20-30 minutes.
03 - Roll chilled dough on lightly floured surface into 10 x 14-inch rectangle. Place chilled butter layer in center of dough. Fold dough edges over butter to completely encase. Pinch seams to seal. Roll out gently to original 10 x 14-inch rectangle, then fold into thirds like a letter. Wrap in plastic and chill for 30 minutes.
04 - Repeat rolling and folding process two more times for total of three laminations. After each fold, wrap dough and refrigerate for 30 minutes. This creates distinct flaky layers in final pastry.
05 - Roll final dough into 12 x 16-inch rectangle. Using sharp knife or pizza cutter, divide dough into 8 equal triangles. Starting from wide end of each triangle, roll tightly toward tip. Curve ends slightly to form crescent shape. Place on parchment-lined baking sheet, leaving 2 inches between pieces.
06 - Cover baking sheet loosely with plastic wrap or clean towel. Place in warm, draft-free area for approximately 2 hours until croissants have doubled in size and feel puffy and light when touched gently.
07 - Preheat oven to 400°F. Whisk egg yolk with 1 tablespoon milk until smooth. Brush egg wash gently over surface of each proofed croissant, taking care not to deflate dough.
08 - Bake croissants for 18-20 minutes until deep golden brown and crispy. Rotate baking sheet halfway through for even browning. Croissants should feel light and sound hollow when tapped.
09 - Transfer croissants to wire cooling rack. Let cool for at least 15-20 minutes before serving to allow internal structure to set and prevent soggy texture.