Basic Homemade Croissant Pastry Dough

Name: Basic Homemade Croissant Pastry Dough
Ingredients: For the Starter Batter (Détrempe):; 2 envelopes (about 4 1/2 teaspoons) dry yeast (or 2 tablespoons fresh yeast); 3/4 cup/175 mL warm water; 3/4 cup/about 100 g flour; 1/2 cup/120 mL warm milk; 2 tablespoons sugar; For the Laminated Dough:; 3 cups/390 g flour; 2 teaspoons salt; 12 ounces/340 g of cold unsalted butter cut into 1/2 inch (1.3 cm) pieces; additional flour for rolling out the dough; egg wash made from 1 egg and 1 tablespoon water