Beat the eggs really well, and then beat in the condensed milk and vanilla until combined.
Beat in the whole milk.
Add the lemon zest and stir.
Coat a flan pan (any normal baking mold will do) with butter and then with caramel.
Pour in the egg and milk mixture, and place in a water bath (bain marie) for about 35-50 minutes (the time will depend on the pan you are using– so after about 35 minutes check frequently).
When a toothpick inserted into the center of the flan comes out clean, it’s ready!
Let the flan cool completely, then flip over onto a serving dish. Spoon extra caramel sauce on top and enjoy!