Beat together the egg and sugar in a bowl. Sift in the flour and mix.
Add milk into the bowl from Step 1 and whisk well without causing it to foam. Cover with plastic wrap and microwave at 700W for 2 1/2 minutes. Mix well, and microwave again for 30 seconds. Mix well and microwave for 20 more seconds.
Take the bowl out of the microwave, add the vanilla extract and margarine, mix well until the mixture is smooth, cover tightly with plastic wrap, and cool in the refrigerator.
Story Behind this Recipe
Custard cream made with only egg yolks is good, but I used a whole egg because I did not want to waste egg whites, and I wanted to make a large amount.
When you take the custard cream out of the microwave, mix well to prevent lumps. You don't need to add the margarine. If the cream is still loose at step 2, microwave a few seconds without the plastic wrap.