Get cranberries and apple cider vinegar to a boil in a large pot.

Mash cranberries thoroughly, then run them through a food mill into another pot.

Stir sugar into the cranberries but don't turn the heat back on. Stir until sugar is dissolved.

Slowly pour in the beaten eggs while whisking constantly so they don't scramble.

Put the butter in a separate pot. Pour in the cranberry mixture over the butter.

Stir until butter is completely melted and incorporated.

Cook on low to medium-low heat, stirring constantly, until the curd is smooth and spoonable. Don't overcook it to the consistency of pudding. It'll thicken up as it cools.

Ladle into half-pint jars, leaving 1/4-inch headspace.

Process in a waterbath for 15 minutes.