Break up the chocolate and put in a bowl with the brandy or rum. Put the bowl over a saucepan of gently simmering water and allow to melt slowly.
Melt the butter in a saucepan, add the flour and cook over a low heat for a few minutes, stirring constantly. Warm the milk in another saucepan, melt the caster sugar in it and then blend in to the flour and butter. Continue to cook slowly, st