Melt the semi-sweet chocolate and unsalted butter over a double boiler. Set aside.

In a pot, combine 3 1/2 cups of milk and sugar.

Bring to a gentle boil over medium heat, then reduce heat to low.

Add the melted chocolate mixture and stir to combine.

In a small bowl, mix corn starch with 1/2 cup of milk to create a slurry.

Slowly add the corn starch slurry to the boiling milk, stirring constantly to prevent clumping.

Continue stirring until the pudding thickens and starts to bubble.

Pour the pudding into 4 serving glass bowls.

Let cool slightly, then refrigerate for at least 30 minutes before serving.