In a small saucepan, heat the water and evaporated milk just until steaming and small bubbles appear at edges. Remove from heat and let cool 5 minutes (to 120-130°F).
In a medium bowl, combine the salt, sugar, yeast and 1 cup of the flour. In the bowl of a stand mixer fitted with a paddle attachment, combine the water and milk with the dry ingredients and butter. Mix on medium speed for 4 minutes. Add the egg and mix until incorporated, about another 1 minute.
Switch the paddle attachment for a dough hook, and gradually add the remaining flour a ¼ cup at a time while mixing on low until you have a soft dough that doesn't stick to your hands when touched. Knead until smooth and elastic, about 5-7 minutes. Form dough into a ball and place in a greased bowl. Cover with plastic wrap and chill in the fridge for 2 hours.
Let come to room temperature for 30 minutes. Knead the dough a few times to relax the gluten. Roll dough out into a 14-inch circle and cut into triangles.
Roll up each triangle starting with the wide end and tuck the pointed end underneath. Curl the roll into a crescent and place on a sheet pan lined with parchment paper or a silicone baking mat.
Let croissants rise in a warm place until an indentation remains when dough is pressed lightly with a finger, about 30-60 minutes. Brush with egg wash (1 egg + 1 teaspoon water) and bake at 350°F until golden, 15-18 minutes.
Let cool on a wire rack. Croissants are best the day they are made but may be stored in an airtight container at room temperature for a few days.