Freeze a bowl (wet it for it to freeze in a shorter time). 

Bring milk to a boil in a medium saucepan, turn off the heat and add salt, lemon zest and the vanilla bean (carved to release beans and aroma). Let it infuse for a few minutes. 

Meanwhile beat together egg yolks and sugar until frothy. Sift the flour and add it to the yolks and sugar mixture. Mix well. Add a few scoops of milk (remove the vanilla bean first) through a fine sieve, and mix well. Add the remaining milk and incorporate it completely. 

Return the crema to the saucepan and cook it, over medium/low heat, stirring continuously until it thickens. 

Pour the crema into the frozen bowl and stir energetically for a minute. 

Ready!