Put the egg, flour and sugar into a heatproof bowl or container and mix well until the lumps disappear.
Pour the milk into the bowl in 3 batches and mix well.
Loosely cover with plastic wrap and microwave (600W) for 2 minutes.
Remove from the microwave, give it a stir, then microwave for 1-1.5 minutes. Give it a good stir again.
Add vanilla oil and mix in. Add 10 g of butter (not listed) if you like to give it shine and richness.
When letting it cool, cover tightly with wrap. When it has cooled down, put it in the fridge.
Use it in a strawberry tart.
Someone asked me how long it keeps. I eat it within 3 days at the most. It still tastes fine, so I would say it's OK for 3 days.
However, this is just a guide. If you are worried, eat it as soon as possible.