Add the finely chopped chocolate and cubed butter to a large heat-safe bowl. Set aside.
Sift the sugar, Rodelle Gourmet Baking Cocoa, cornstarch, and espresso powder into a large saucepan to remove any lumps, then whisk until well combined.
In a separate mixing bowl, whisk together the whole milk, heavy whipping cream, and egg yolks until fully combined. Pour about 1/2 cup of the wet ingredients into the saucepan with the dry ingredients and whisk until well combined, then pour in the remaining wet ingredients and whisk until fully combined.
Place the saucepan over medium heat, stirring continuously and scraping the bottom and sides of the pan often, until the mixture starts to thicken. Once the mixture thickens and comes to a boil, allow to boil for one minute, then remove it from the heat.
Strain the filling through a fine mesh strainer into the bowl with the chopped chocolate and butter. Once strained, stir until the chocolate and butter are fully combined, then stir in the vanilla extract.
Scoop the filling into the cooled pie crust and spread it around into one even layer. Place a piece of plastic wrap directly on the surface of the filling and allow to cool slightly for about 15 minutes at room temperature, then transfer to the refrigerator to chill for at least 3-4 hours.