A full batch of pancake (1 egg, 1-1.5 cups flour) is a too much for me so I wanna split it into 2 half batches. How should I split and store the ingredients? Can I mix the butter, egg, salt and sugar and store half that, only mixing in the flour, baking soda & vinegar when ready to cook?

I'm mostly worried about melted butter/margarine and egg since I don't know how they last

can't you just make the pancakes and freeze them?

^ this would be my suggestion as well. Freezing is optional. In our house, pancakes tend to sublimate anyhow.

I don't eat eggs but wouldn't the eggs contribute a bit to trapping air too?

so stroring the batter may make it go flat

@JourneymanGeek Not for pancake batter, which usually uses the eggs unwhipped, just stirred. The bubbles are created by the chemical leavener, then “trapped” by the coagulating egg and flour. But I don’t think the previously-melted butter would fare too well in the fridge.

@JohnZhau food safety-wise, no problem for a few days in the fridge, but I don’t think it’s a good idea because of the butter. A baked pancake (or crepe) stores quite well, both at room temperature or frozen and can even be reheated if necessary.