I'm making a birthday cake for a woman who loves anything old. I've tried to find info on 'tea' staining online but nothing I'm pretty sure I don't want to airbrush because I'm not that experienced with details. I think I read something a while ago about someone using vanilla extract on fondant? I'd love to cover the cake in buttercream icing but not sure if it's paintable? Any suggestions would be greatly appreciated!!!

I read a tutorial not that long ago (forget where tho! maybe half baked? the cake was teal blue). What they did was liberally brush on a colour (in your case a tan/brown) and then blot most of it off with a paper towel. I think this would give you the teas stained look.

A light dusting of cocoa powder should work

Was it a dry brush or color or a wet color?

it was wet - i think maybe even just get colour diluted a smidge with vodka, but still pretty thick

I'm going to try the cocoa first on a test piece!! Thanks for the quick responses)

If your buttercream is cold and hard, you can paint on it with no problem. I think sponging with a very light brown color would give you a lovely effect. Perhaps use two colors for more depth. Gel color thinned with lemon extract (it dries faster than vodka).