In a medium saucepan, combine cocoa, corn starch, espresso powder, salt, sugar, vanilla and egg yolks and whisk to combine. Â Slowly add milk and cream and whisk until mixture is smooth.
Bring to a simmer over medium heat, whisking constantly until the custard is very thick, about 5 minutes. Remove from the heat, add the chopped chocolate and butter and whisk until smooth.
Divide custard among 4 coffee cups or ramekins. Set aside to cool to room temperature and then cover with plastic wrap and refrigerate until chilled.
4 servings
3/4 cup