I could try a hotter oven, see if that makes the difference. It never used to happen with my old oven, not sure I was using this recipe then though.
I usually take them from the tins after maybe a minute, it's definitely oil though, not condensation. I use a baking paper circle in the bottom of tins, or in this case individual paper cases & they're all definitely soaked with oil. At first I thought it was because I'd tried using alternatives to butter (like Lurpak spreadable) as it was softer & that maybe the oil content caused it, but it happened this time in the individual cases & last time in the tins when I used all butter.
I used to butter & flour tins before I bought these circles, now I just wipe the edges round with a butter paper or a kitchen tissue with a little oil on, so there's no visible residue at all. I didn't oil the paper cases at all of course.
I think I'd rather fix it than abandon butter altogether at this stage, particularly as they still taste fine & when it is a whole cake no one knows but me I guess! It's more of a problem with cupcakes though, unless I change the cases! Maybe I've not noticed before as I sometimes use the silicone ones. I think I'd probably miss the flavour of butter a lot. I've not loved any of the oil cakes I've tried. No harm in trying another though.
West Sussex, not the arctic! Just have a cold house, barely any heating in winter & efforts to shade & keep it cool in the summer. Both us humans & even more so our siberian husky like it cool. Baking however, doesn't!