In a medium saucepan, pour the whole milk and heat it over medium heat until it's hot but not boiling. You’ll notice small bubbles forming around the edges. Keep an eye on it and stir occasionally.

While the milk is heating, whisk together the egg yolks, sugar, cornstarch, and salt in a separate bowl. Make sure it’s smooth and well combined.

Once the milk is hot, slowly pour a small amount of the hot milk into the egg yolk mixture while whisking constantly to prevent curdling. Gradually add more milk to the egg mixture, continuing to whisk as you go.

Now pour the tempered egg mixture back into the saucepan with the remaining milk. Cook this mixture over medium heat, whisking constantly until it thickens, about 3 to 4 minutes. Be sure to stir the edges and bottom of the pan to prevent any lumps.

Once the mixture thickens, remove it from the heat. Stir in the chopped chocolate, butter, and vanilla extract. Whisk until smooth and the chocolate has fully melted into the cream.

For an ultra-smooth texture, you can strain the cream through a fine mesh strainer into a clean bowl. Let it cool slightly, then cover with plastic wrap, making sure it touches the surface of the cream. This will prevent a skin from forming. Chill in the refrigerator for at least 1 hour before using.