The biggest thing with rum cake is balancing moisture and sweetness without making it soggy or overly heavy.
A few things that really help:
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Use buttermilk, sour cream, or pudding mix in the batter. They keep the cake soft for days instead of drying out after a few hours.
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Don’t overbake it. Rum cake keeps cooking a little after it comes out of the oven, so pulling it out as soon as a toothpick is mostly clean makes a huge difference.
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The rum flavor usually comes more from the glaze than the batter itself. A warm butter-rum syrup poured over the cake while it’s still warm gives the best flavor.
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Pour the glaze slowly in layers so the cake absorbs it instead of having it run off the sides.
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Brown sugar instead of only white sugar gives a deeper flavor and makes the sweetness feel less sharp.
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Toasted pecans or walnuts add texture and also balance the sweetness nicely.
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Let it sit overnight if possible. Rum cake honestly tastes better the next day because the flavors settle into the cake.
The best rum cakes taste rich and buttery with a noticeable rum flavor, but not so sweet that you can only eat one bite.