Peel mangoes with either a small sharp knife or potato peeler, then halve them by slicing the lobes off each side of the stone. Lay mangoes on a board, cut-side down, and cut in half horizontally. Place on an oiled, foil-lined baking tray and sprinkle with demerara sugar. Place under a grill, about 3–5 cm (1 ¼–2 in) from the heat, and cook until sugar caramelises and mango begins to colour. Leave to cool.
Place a small teaspoon of the cream on four plates and sit a wafer on top of each. Place an eighth of the cream on each wafer and smooth it out, then sit the flatter pieces of mango on top. Sit a wafer on these and again spread some banana cream on top, sit the rounded pieces of mango on next, then balance the last wafers on top. Serve with a lime wedge to complement the taste of the mango and to cut through the richness of this dessert.