Place the chopped dark chocolate in a microwave-safe bowl. Heat in 20-second bursts, stirring between each, until fully melted.
Stir the cooled espresso into the melted chocolate until smooth. Set aside to cool slightly.
In a mixing bowl, whip 1 cup of the heavy cream to soft peaks. Gently fold the whipped cream into the chocolate mixture until no streaks remain.
Divide the mousse among four small cups or glasses. Place in the refrigerator for at least two hours to set.
In a clean bowl, combine the mascarpone, powdered sugar, and remaining 1/4 cup heavy cream. Whip until the mixture holds soft peaks.
Spoon the whipped mascarpone over the chilled mousse. Dust each cup lightly with cocoa powder before serving.