This is the BEST and easiest chocolate pastry cream! You make it in one pot and there's no need to temper the egg yolks. It's thick, rich, glossy and luscious with intense chocolate taste. Use this fill cakes, cupcakes and pastries or serve it alongside pie with some fresh berries. It makes a fabulous dessert on its own with some crushed biscuits or meringue with fresh fruit.
In a small 1-quart stainless steel saucepan, combine sugar, corn starch and cocoa powder and whisk to blend evenly so there are no lumps of starch or cocoa. Very slowly begin to whisk in the cream so that it blends smoothly with the starch/sugar mixture. Once all the cream is added, you can pour in the milk and whisk well. Add egg yolks one a time and whisk them in evenly before each addition.
Place the sauce pot over medium-low heat and whisk constantly until it starts to thicken. Be patient as this can take up to 10 minutes. Increase the heat slightly to medium if necessary, but do not walk away from the pan and do not stop stirring as it can easily catch and scorch at the bottom of the pan. Once the mixture starts to thicken, lower the heat and watch carefully. Once it starts to bubble, let it bubble for just 5 seconds while stirring then immediately transfer it to a clean bowl. NOTE: you can pass it through a fine mesh sieve to remove any curdled egg pieces if you are so inclined.
Add the chopped dark chocolate and whisk until it is completely melted and evenly combined. Add butter and vanilla and stir it in until melted and evenly incorporated. Place a piece of plastic wrap directly over the surface of the custard to cover it and refrigerate for at least 4 hours until thoroughly chilled and use within 3 days.