Preheat the slow cooker if necessary; see the manufacturer's instructions. Put the butter and sugar in a large basin, checking first that it will fit into the slow cooker pot, then heat in the microwave until the butter has just melted. Alternatively, heat the butter and sugar in a saucepan and pour into the basin.
Stir the sugar mix, then gradually whisk in the eggs, and then the fruit rind and juice. Cover the basin with foil and lower into the slow cooker pot. Pour boiling water into the pot to come halfway up the sides of the basin. Cover with the lid and cook on low for 3–4 hours, stirring once during cooking, until thick.
Stir once more, then scoop the passion fruit seeds out of the halved fruit with a teaspoon and stir them into the preserve.
Warm 3 clean jars in a low oven for 5 minutes. Ladle the preserve into the warm jars, cover the surface with waxed discs, add cellophane jam pot covers and secure with elastic bands or screw the lids in place. Label and leave to cool. The preserve can be stored for up to 2 weeks in the refrigerator.