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 butter into rectangle approximately 5 x 7 inches. Refrigerate until firm.
03 - Roll chilled dough on lightly floured surface into 10 x 14-inch rectangle. Place butter layer in center. Fold dough over butter to completely encase. Gently roll out to 10 x 14-inch rectangle. Fold into thirds like a letter. Wrap and refrigerate for 30 minutes.
04 - Repeat rolling and folding process two more times for total of 3 folds, refrigerating 30 minutes between each fold.
05 - Roll dough into 12 x 16-inch rectangle. Cut into 8 triangles. Starting from wide end, roll each triangle into croissant shape. Arrange on parchment-lined baking sheet, seam side down.
06 - Cover croissants loosely with plastic. Let rise in warm place for approximately 2 hours until doubled in size and puffy. Preheat oven to 400°F. Whisk egg yolk with milk for glaze. Brush evenly over risen croissants. Bake for 18-20 minutes until deep golden brown.
07 - Transfer croissants to wire rack. Cool for at least 15 minutes before serving to allow interior structure to set.