Leash it's similar, but my recipe uses 2x whole eggs rather than just the yolk. Then it says to beat the eggs & sugar together while heating but not boiling the milk. Slowly mix those together then heat again while stirring constantly until it thickens. Let it cool to room temperature then add in the vanilla & cream. It has to be cooled in the fridge for 12 hours as well.

A thought I had was that maybe the custardy mix you made was lumpy?

And for you Janie, but I'll let you know how it goes when I make it.
Ingredients:
1/2 pint (250ml) double (heavy) cream
1/2 pint (250ml) single (light) cream
2oz (50g) sugar
1/2 cup peanut butter (chunky or smooth - you choose)
1/2 teaspoon vanilla extract

Place the cream, sugar and peanut butter into a small/medium saucepan and heat gently. Stir all the time whilst heating. When smooth and the sugar has all dissolved, remove the saucepan and leave the mixture to cool. When cool, gently stir in the vanilla extract.

Transfer the mixture into an ice cream maker and freeze following the manufacturer's instructions.