Whisk together the flour, sugar, yeast, and kosher salt in a large mixing bowl.
Cut the cold butter into the flour mixture using a pastry blender or two knives until only pea-sized pieces remain. Keeping the butter cold helps create flaky layers.
In a separate bowl, whisk together the milk, water, and egg. Add the mixture to the flour and butter and stir just until a dough forms.
Wrap the dough in plastic wrap and refrigerate for at least 30 minutes.
Lightly flour your work surface and roll the chilled dough into a rough rectangle about ½ inch thick. Fold it into thirds like a letter, turn it 90 degrees, roll it out again, and fold. Repeat the rolling and folding process 3–5 times.
Wrap the laminated dough and refrigerate for another 30 minutes. Divide it into 3 equal portions.
Roll each portion into a rectangle about ⅛ inch thick, approximately 16 inches long and 5 inches wide. Cut each rectangle into skinny triangles.
Starting at the wide end, roll each triangle toward the pointed end. Tuck the point underneath and gently curve the roll into a crescent shape. Arrange on parchment-lined baking sheets.
Cover loosely with plastic wrap and let the rolls rise for about 1 hour, or until puffy and nearly doubled in size.
Preheat the oven to 400°F. Bake the crescent rolls for about 10 minutes, or until puffed and golden brown.