Store 2 kgs powdered milk
Store 4 kgs powdered milk
Powdered milk again. Make sure you check the use-by date on your powdered milk. I once purchased some that had to be used up within 2 months. I use a tablespoon of powdered milk each time I make bread, but I didn't get to make that much bread!
Other options if you already have enough powdered milk, or if you don't wish to store it are sweetened condense milk, or evaporated milk. I like to keep both of those in stock too, as I use them whenever I make Nigella Lawson's Flan (aka Creme Caramel). It gets devoured quickly here, and I confess, I could eat an entire thing by myself. Well, almost by myself. Over two days perhaps ;D