Place the mascarpone in a small bowl. Add the espresso, cocoa, and sugar. Stir well to blend thoroughly. Cover with plastic wrap and refrigerate at least 30 minutes to develop flavors. Remove from the refrigerator a few minutes before serving.
Place the bread rounds on a baking sheet and toast them in a 350 oven until lightly golden on the outside but still soft within.
If desired, lightly butter the crostini. Then, using a knife, spread the mascarpone mixture on the crostini, smoothing it into low mounds. Dust the tops lightly with cocoa.