I make all of my desserts and baked goods from scratch (2019: still true for desserts, not always for bread), so that keeps my baking cupboard pretty simple. Even if you use mixes now, I would recommend only storing basic baking supplies in your 3 month supply to save both money and space. The only thing it won’t save you is time, but once you cook a recipe a few times, it will be almost as easy as using a box.
Here’s what I keep in my baking cupboard:
Chocolate chips: There is no way to store these for 3 months because they’re way too tempting, but I always have some on hand.
Sugars: brown, white, and powdered in reused #10 cans
Vanilla: I splurge for the real stuff and buy a larger container online. I just don’t like the taste of imitation.
Baking soda: large box because of its usefulness as a cleaner in addition to baking
Salt, regular and kosher
Oils: canola, non-stick spray
Yeast: I also buy this online by the pound because my local store stopped selling it. It lasts forever in the freezer!
Baking supplies are so important because of how versatile they are.