Wash the cassava by leaving it to soak for 2 minutes in water. Strain.
In a bowl, blend the egg yolk with 1 tablespoon of growing-up milk formula. Set aside.
In a saucepan, cook the rest of the growing-up milk formula and butter on a low heat.
Once the butter has melted, add the cassava and vanilla and cook until it thickens, stirring all the while.
Remove the pan from the heat and then add the egg/milk mixture. Heat again on a low heat stirring all the time. Serve.