Mix flour, sugar and salt in the top of a double broiler (or put one pot inside of another with water in the bottom pot).

Heat up milk in the microwave for 2 minutes.

Add milk gradually, then add in chocolate. Cook until thick, stirring constantly.

Add a small amount of the hot mixture to the egg yolks (to get the temperature of the eggs up so that they don't scramble in the pudding- this is called tempering). Then stir this mixture into the rest of the hot mixture.

Cook for 2-5 more minutes, then cool. When almost cool, stir in vanilla and butter.