01 - Whisk together gluten-free flour, sugar, salt, and instant yeast in a large mixing bowl. Combine warm milk and egg separately, then pour into dry ingredients. Mix until soft dough forms, then beat in softened butter until fully incorporated. Cover bowl with plastic wrap and refrigerate for 1 hour.
02 - Place cold butter between two sheets of parchment paper. Using a rolling pin, pound and roll butter into a 5 x 7-inch rectangle. Transfer to refrigerator and chill until firm, approximately 20 minutes.
03 - Roll chilled dough on lightly floured surface into 10 x 14-inch rectangle. Position chilled butter layer in center. Fold dough edges over butter to completely encase. Roll out gently to 10 x 14-inch rectangle, then fold into thirds like a letter. Wrap and refrigerate for 30 minutes.
04 - Repeat rolling and folding process two additional times for three total folds, chilling 30 minutes between each fold. After final fold, wrap dough and refrigerate for 1 hour.
05 - Roll dough into 12 x 16-inch rectangle. Using sharp knife or pizza cutter, divide dough into 8 equal triangles. Starting from wide end, roll each triangle toward tip, forming traditional crescent shape. Place shaped croissants on parchment-lined baking sheet, seam side down.
06 - Cover baking sheet loosely with clean towel or plastic wrap. Place in warm, draft-free area for 2 hours until croissants have doubled in size and feel puffy to the touch.
07 - Whisk egg yolk with 1 tbsp milk until smooth. Brush mixture gently over each croissant, avoiding excess pooling that may inhibit rise.
08 - Preheat oven to 400°F. Bake croissants for 18-20 minutes until deep golden brown with visible flaky layers. Rotate pan halfway through baking for even coloring.
09 - Transfer baked croissants to wire cooling rack. Let cool for at least 15 minutes before serving to allow interior structure to set.