Can I cream butter, sugar and eggs ahead of time
You can cream them first, but it will be better to let them sit in the bowl while you go fetch the extract. If the store is far and your round trip time will be greater than an hour, I would put the ingredients in the refrigerator. Otherwise, I would leave them on the counter (cover if your kitchen is dusty or prone to small objects falling into bowls or insects) and cream them once you return. If you refrigerate the ingredients you'll have to bring them back up to room temperature if you want to cream them perfectly (however for most home cooking and baking cold temperature creaming works just fine as well - just not the best results).