Heat 400ml of the milk in a small saucepan with the vanilla bean. Heat gently for 4 minutes, and do not allow to boil. Remove from heat.
In a bowl, whisk together the egg yolks and sugar till smooth and pale. Combine the flour and cornflour and whisk into the egg mixture till smooth, then add the remaining 100ml cold milk and whisk well.
Remove the vanilla pod from the warm milk. Add the egg yolk mixture to the warm milk in the saucepan, then whisk well. Place over a low heat and stir constantly for 6 to 8 minutes, till thickened.
Remove from heat and transfer the pastry cream to a bowl. Dab the surface with the butter to prevent a skin from forming. Transfer to the fridge and chill for at least 30 minutes, or till ready to use.