Sift together flour, dried powdered buttermilk, sugar, baking powder, baking soda, and salt.
Refrigerate this "from-scratch mix" in plastic storage bags until you’re hungry for waffles. (It will keep for 1 month.)
On a waffle morning, whisk eggs in a large bowl until blended, then whisk in water and melted butter.
Whisk dry ingredients into liquid mixture just until smooth.
Brush a preheated waffle iron lightly with vegetable oil and spoon batter—about 2 cups for 4 waffles—in iron, spreading quickly.
Cook waffles according to manufacturer’s instructions, transferring as cooked to rack of a preheated 250°F oven to keep warm. (If you don’t intend to use the entire mix at once, divide dry-ingredient mixture into thirds and transfer to 3 sealable plastic bags to refrigerate. You will need 1 egg, 1 cup water, and 2 tablespoons melted unsalted butter for each bag.)
