Combine the cream and vanilla in a medium saucepan and bring just to boil , then leave to infuse for 15 minutes , meanwhile lightly beat the egg yolks and half the sugar until well combined . After the 15 minutes , strain infused cream over the beaten yolk mixture and whisk briefly to blend then return to same cleaned saucepan and stir over low heat until custard thickens enough to coat the back of a spoon .