Place egg yolks in a bowl. Add sugar.
Whisk vigorously to combine.
Sift in the flour a little at the time to avoid lumps. Whisk thoroughly until fully combined before adding more. Combine vanilla.
Put the milk in a saucepan on medium heat. Add the slice of lemon rind. When the milk comes to a boil, remove the rind. Stir slowly and steadily the milk into the egg mix.
Transfer to a saucepan on medium-low heat. Bring slowly to a boil stirring constantly in the same direction. Do not over-boil, but keep on the stove until the cream starts thickening.
Remove from the stove and let cream cool at room temperature. Stir occasionally, to avoid the formation on the surface of a thicker patina.