To make the espresso cream

Place the egg yolks and sugar in a medium-sized bowl. Whisk until thick. Stir in the espresso, the milk, and then the cornstarch.

Transfer the espresso milk to a noncorrosive heavy-bottomed saucepot. Stirring constantly with a rubber spatula, cook the mixture over medium low heat for about 10 minutes, until thick. Be sure to scrape the bottom of the pot or the espresso milk will stick and burn. Remove the pot from the stove and stir in the chocolate and vanilla extract.

Strain the espresso mixture through a strainer. Transfer it to a stainless steel bowl. Place plastic wrap directly on the surface to prevent a skin from forming. Refrigerate the espresso base until cool.

Put the heavy cream in the bowl of an electric mixer. Using the whisk attachment, whip on high speed until soft peaks form. Fold it into the espresso cream. Refrigerate the cream until you are ready to assemble the trifle.

For the Cognac syrup

Stir together the simple syrup and the Cognac. Set aside.