Peel the custard apple and remove all the seeds. Combine it in a food processor with the sheep's yoghurt and vanilla.
Pour the cream into a freezer container and place in the freezer for 2 hours.
Remove from the freezer, churn it in the food processor once more and return it to the freezer.
In a large frying pan, add the maple syrup to the chopped walnuts.
Stir over a medium heat until the walnuts brown slightly.
Remove from the heat, separate any nuts that have stuck together and set aside to cool.
Remove the ice cream from the freezer, return to the processor and churn it once again. Stir through the walnuts, and return it to the freezer. Serve with baked apple rings.
Note: Select hard custard apples and leave to ripen. Once soft, pull the fruit apart from the hole where the stem was and remove the segments with your fingers or a small spoon. Discard the skin and seeds.