Melt the butter in a small saucepan over low heat. Whisk in the flour. Gradually stir in the pareve milk. Cook, stirring constantly, until thick and smooth. Add chocolate; continue to stir until melted. Stir in the coffee. Remove from heat. Stir in the vanilla and 1/4 cup of the sugar. Add the egg yolks, one at a time, beating well after each addition.