Assume I have a decent waffle recipe that I like. Assume it has nothing particularly unusual for waffles -- this means it has liquid ingredients such as milk or buttermilk, eggs, butter, vanilla extract. How can I modify this to make an "instant" waffle mix where I just add one or two ingredients (say, milk)? Can I simply replace most of the liquid ingredients with powdered or solid versions? Or must other adaptations be made to compensate? Will I have to adjust proportions?
If you replace liquid ingredients with a powdered equivalent, you'll need to add in additional liquid to compensate.
For instance, let's say the recipe calls for two eggs. If you replace two eggs with a powdered substitute, you'll need to add water to the mix in addition to the milk in order to replace the moisture loss by using a powdered egg substitute.
I'd also be concerned about loss in product quality when using an instant / powdered substitute, but in principle it should work.
That being said, I work in a bakery where we do something similar. For our muffins and scones, we combine the butter, flour, salt, sugar and baking powder and mix it until the butter is incorporated into the dry ingredients. We also beat together the eggs, milk and water required.
The dry mix and the wet mix are stored separately in large quantities in refrigeration and when we go to bake we simply combine them.
There will be a shorter shelf life to the dry mix because it's got the butter in it, but under refrigeration it lasts a long time. In theory, you could store it in the freezer -- I know you can freeze butter with minimal loss of quality so I cannot think of a reason why this wouldn't work, and it will give it a very long shelf life (however long you can freeze butter for.)
The egg, milk & water mixture's expiration date is whichever expires first from the egg and milk.
This probably isn't a practical solution unless you're making waffles all the time, but if you make them a few times it week it might be worth it because you won't lose quality. Otherwise, parse through your waffle recipe an ingredient at a time, replacing liquid with powdered versions, making sure to keep track of the water you'll need to add back in.