Break egg into a double boiler or heavy bottom saucepan, whisk lightly. Add remaining ingredients.
Cook on medium heat, whisking frequently until mixture starts to bubble. Cook for a few minutes, whisking constantly, until thick. Do not boil at high heat or you are likely to overcook your egg and cream.
Pour into a heat proof container (I use pyrex); cover and refrigerate. Mixture will thicken more as it cools.
Makes around 2 1/2 cups. Keeps for about a week in the fridge. Also goes well with other firm fresh fruits such as pears or not too ripe bananas, and makes an excellent ice cream topping.