Stir the water and espresso powder in a large bowl to blend. Stir in the mascarpone. Using an electric mixer, beat the cream in another large bowl, while slowly adding icing sugar, until soft peaks form. Using a large rubber spatula, fold 1/4 of the whipped cream into the mascarpone mixture to lighten. Then fold the remaining whipped cream into the mascarpone mixture.
Cut the angel food cake into 16 (1/2-cm thick) wedges. Reserve any remaining cake for another use. Brush 1 side of each wedge of cake with the liqueur. Arrange 2 wedges of cake on each plate. Dollop the espresso cream atop each wedge of cake. Dust with the cocoa powder and serve.