Heat the milk on medium-low heat until it starts to steam with small bubbles around the edge. Turn off the heat.

Whisk egg yolks with sugar in a bowl until the mixture is a light yellow color.

Add the corn starch, cocoa, and salt to the egg mixture and whisk thoroughly, making sure there are no lumps.

Add about a half cup of the warmed milk to the egg mixture, whisking vigorously to temper the eggs. Add the egg mixture to the milk and incorporate thoroughly.

Cook on medium-low just until the mixture starts to bubble. Be sure to frequently stir or the pudding will start to burn at the bottom. Lower the heat to simmer and cook for five minutes, stirring often.

Once the pudding is thickened, turn off the heat and stir in the butter and vanilla.

After the butter has melted, add in the chopped chocolate and stir until it is thoroughly melted and incorporated.

Cover with plastic wrap, being sure to let it sit directly on top of the pudding to avoid a skin forming.

Refrigerate for at least two hours.

Serve with whipped cream.