Put the milk in a heavy-based saucepan and add the lemon rind and vanilla. Bring just to the boil. Meanwhile, whip 2 eggs in a bowl until they are creamy, then whisk in the sugar. Add the flour and whisk to a smooth cream. Just as the milk comes to a rolling boil, whisk a ladleful into the eggs. Whisking constantly, add another ladleful or two of milk. Spoon it all back into the milk pan and put over the lowest heat, whisking all the time. It will thicken quickly so you may have to remove the pan for a minute and whisk vigorously until it’s smooth. Put back on the heat for a few minutes to cook the flour, whisking until it’s completely smooth and very thick.
Lightly grease a 17 × 27 cm(6½ × 11 inch) sandwich tin. Spoon the custard into the tin, smoothing the surface with a spatula. Let it cool and set completely, then turn out and cut into 5 cm(2 inches) squares. Break the last egg into a small flat bowl and scatter some flour on a plate. Melt 2–3 tablespoons of butter in a non-stick frying pan over medium–low heat. Working in batches, dip squares of custard first in the egg and then in the flour to coat lightly. Fry in the butter until golden on both sides, turning them gently. Add blobs of butter along the way to prevent burning. Lift the squares out onto kitchen paper to drain and then serve warm, sprinkled with some caster sugar.