Sunk muffins

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www.bbc.co.uk/food/recipes/lemon_muffins_01522

We used this recipe today - firstly, there was a lot more batter than we thought there would be, but it was the right texture for a muffin mix. But sadly, they didn't work at all. There was a lot of oil in the muffin tin when we took them out, and one batch totally sank after we took it out. Any ideas what we may have done wrong?

Were they definitely cooked when you took them out? Did you test them?

My gut feeling is that the recipe seems like it has a lot of liquid and I haven't used a recipe like that.
melted butter, oil, milk and eggs and there's lemon juice. Adds up to a lot of oily liquid

I googled lemon muffins to check other recipes and this seems to have a lot less liquid. Way less butter and yoghurt instead of milk

prettysimplesweet.com/lemon-muffins/

Sorry I meant had less butter and no oil. I would tend to only use one or the other.

Thanks very much. The mixture didn't look too runny, but I've not made muffins for a few years so may be misremembering. I will give the other recipe a try, thanks @cravingthelook.

Tested them with a metal skewer and it came out clean. That's all I ever do to check cakes etc.