Divide the brownie or cake pieces between 4 sundae glasses. Melt the mints very gently, for about 20 seconds, in a bowl in the microwave. Add the boiling water and stir until thick. Take about one-third of this warm chocolate fondant and spoon it over the brownie or cake pieces in each of the serving glasses. Whip the cream until thickened but not too stiff, then fold in the remaining chocolate fondant together with the crème fraîche. Divide the chocolate mint cream equally between the sundae glasses, spooning it over the layers of cake and mint cream fondant. Finish with a thick dollop of crème fraîche and a sprinkling of grated chocolate. Cover loosely with foil or clingfilm and chill the sundaes for at least 30 minutes before serving.