Prepare 150ml espresso, then add cold milk to reach a total volume of 300ml.

In a saucepan, combine sugar and corn starch, then add the cold coffee milk and mix thoroughly before stirring in the dark chocolate.

Cook the mixture over medium-low heat, stirring constantly, until it boils, thickens, and becomes gooey; then remove from heat.

Pour the hot mixture into moulds or serving glasses, let it cool to room temperature, then cover and refrigerate until fully set.

Prevent water condensation from softening or ruining the jelly.

Optionally, serve the cooled jelly with Kahlúa as a sauce.