First prepare your paste to thicken up the custard. Add to a small bowl: 5 tbsp cornstarch, 1 tsp vanilla and 5 tbsp oatmilk. Whisk until fully smooth.
Grab a medium sized saucepan and put the heat medium-low. Add the remaining vanilla, your coloring of choosing and lemon zest. If you want to add a little freshness, squeeze in half of the juice as well.