01 - Combine gluten-free flour, sugar, salt, and instant yeast in a large mixing bowl. Whisk warm milk and egg together, then pour into 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 a 5 x 7-inch rectangle. Transfer to refrigerator until firm.
03 - Roll chilled dough on lightly floured surface into a 10 x 14-inch rectangle. Position butter layer in center and fold dough edges over to completely encase. Roll gently back to 10 x 14-inch rectangle. Fold into thirds like a letter. Wrap and chill for 30 minutes.
04 - Repeat rolling and folding process two additional times for three total folds. Chill for 30 minutes between each fold.
05 - Roll dough into a 12 x 16-inch rectangle. Cut into 8 equal triangles. Starting from wide end, roll each triangle into croissant shape. Place on parchment-lined baking sheet, curved ends pointing toward each other.
06 - Cover loosely and let rise in warm place for approximately 2 hours until puffy and doubled in size.
07 - Preheat oven to 400°F.
08 - Whisk egg yolk with milk. Brush mixture evenly over proofed croissants.
09 - Bake for 18 to 20 minutes until deep golden brown and flaky.
10 - Transfer to wire rack and let cool completely before serving.