In a bain-marie, melt the chocolate. Mix it with the cereal. Press lightly into 7cm-diameter ring moulds set on a tray lined with parchment paper. Set aside in the fridge.
To prepare ice cream
Cream sugar and egg yolks.
In a saucepan, combine milk and peanut butter and bring to a boil. Pour half the milk mixture onto the egg yolk mixture and whisk until well-combined.
Add the rest of the mixture and cook, stirring continuously over low heat, till 83°C, and the mixture can coat the back of a spatula. Transfer immediately into a mixing bowl and refrigerate until cold.
Whip the cream to the stiff peak stage.
Mix milk mixture with whipped cream and pipe into 7cm half-sphere or muffin silicon moulds. Freeze till solid.
Remove ice cream from moulds, and place on top of the chocolate crisp bases. Serve plain or with caramel sauce.