I used regular liners (as my boss had said the greaseproof liners I was using kept pulling away) and my cupcakes came out with grease in the pan. All my ingredients were at room temperature. What happened?
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Only thing I can think of is that you used too much grease when greasing the pan? I've never seen pools of grease left over in the muffin pan after baking muffins or cupcakes.I don't know what else it could be.
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It might be the recipe.
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What kind of method is used to make the cupcakes?
I'm wondering if there's some kind of situation that is causing the butter to leech out before it sets in the cake - either from improper mixing/creaming, oven temperature issues, the recipe, or something else. If you post the ingredients & method it might help.
Maybe your butter is too soft.
I think Dona's suggestion is the most likely culprit. You want the butter to be just soft enough that you can work it into the sugar, but not much softer than that.
Too much fat