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1
Sterilize jars and lids .
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2
Freeze several small plates to use for testing butter.
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3
Scrape seeds from vanilla bean into a 5- to 6-quart heavy pot.
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4
Add pod and stir in remaining ingredients.
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5
Slowly bring to a rolling boil over moderate heat (this will take about 15 minutes), stirring frequently.
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6
Boil, uncovered, stirring frequently, until plums are tender, about 5 minutes.
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7
Discard pod.
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8
Puree plums with liquid in batches in food mill set over a bowl.
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9
Transfer puree to pot and simmer over low heat, stirring and scraping bottom of pan frequently, until very thick, about 1 1/2 to 2 hours.
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10
(To test for doneness, drop a spoonful of plum butter on a chilled plate, then tilt; the mixture should not be runny.
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11
It should be about as thick as jam.)
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12
Drain jars upside down on a clean kitchen towel 1 minute, then invert.
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13
Ladle plum butter into jars, leaving 1/4 inch of space at top, then run a thin knife between plum butter and jar to eliminate air bubbles.
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14
Seal, process, and store filled jars , boiling plum butter in jars 10 minutes.
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15
Let plum butter stand in jars at least 1 day for flavors to develop.
