Place pureed blueberries in slow cooker and turn to low. After 1 hour stir blueberry puree and prop open lid with a spatula or wooden spoon. After 4 more hours, add spices, sugar, and lemon zest. If it hasn’t thickened much, remove lid and cook on high for 1 hour (note- I skipped this step and it turned out fine). Place butter in food processor or blender and process until smooth. Store in air-tight container in fridge or in processed jars (see below).
Pour into sterilized jars, leaving 1/2 inch of head space, wipe rims and screw on lids. Process in boiling water canner for 10 minutes. Store jars in cool, dark place.
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