1. Heat the milk in a saucepan, but do not let it boil.

2. In a separate bowl, whisk together the egg yolks and sugar.

3. Slowly pour the warm milk into the egg yolk-sugar mixture, stirring well.

4. Pour the mixture back into the saucepan and heat over low heat, stirring constantly, until it thickens.

5. Remove the saucepan from the heat and add the chocolate, butter, and vanilla extract. Stir well until everything is melted and well combined.

6. Pour the pudding into dessert glasses or bowls and refrigerate for at least 2 hours to cool and set.

7. Garnish with whipped cream or fruits before serving.