Combine butter and 1/3 cup flour in bowl; beat at medium speed, scraping bowl often, until well mixed.
Line baking sheet with waxed paper. Spread butter mixture into 12x6-inch rectangle on waxed paper. Place another piece of waxed paper over butter mixture. Refrigerate about 1 hour or until firm.
Combine milk, sugar and salt in 1-quart saucepan. Cook over medium heat 4-5 minutes or until sugar is dissolved. Cool to lukewarm (105°F to 115°F).
Place warm water and yeast in bowl; stir until dissolved. Add cooled milk mixture, 2 cups flour and egg. Beat at medium speed, scraping bowl often, until well mixed. Stir in enough remaining flour to make dough easy to handle.
Turn dough out onto lightly floured surface; knead about 5 minutes until smooth and elastic. Cover; let rest 10 minutes.
Roll dough into 14-inch square. Place chilled butter mixture onto one-half of dough. Fold over other half of dough; seal edges. Roll into 21x12-inch rectangle. Fold into thirds; seal edges. Cover with plastic food wrap; refrigerate 1 hour. Repeat rolling and folding dough 3 times. Cover; refrigerate 3 hours or overnight.
Roll out dough into 24x14-inch rectangle. Cut rectangle in half lengthwise; cut into 6 even sections, making 12 (7x4-inch) squares. Cut each square diagonally, making 24 triangles. Roll up each triangle loosely, starting from wide edge. Place point down onto greased baking sheets; curve ends. Cover; let rise about 30 minutes or ntil double in size.
Heat oven to 375°F.
Beat egg yolk and water in bowl; brush over croissants. Bake 13-16 minutes or until golden brown.
Variation: Chocolate Scones: Instead of cutting squares into triangles, cut each square in half. Place small piece of chocolate in center, folding each long side up and over chocolate, sealing edges. Place seam-side down onto greased baking sheet. Cover; let rise about 30 minutes or until double in size. Brush with egg yolk mixture. Bake as directed above.