How to Make It
Using a sharp knife, peel the oranges, removing the skin and pith so the flesh is exposed. Cut between the membranes of the orange and remove the segments (you will have about 25). Squeeze the membranes over a small bowl to extract all the juice (about 1/3 cup), and reserve the juice.
Mix the mascarpone cheese with the sugar.
Arrange the slices of pound cake side by side in the bottom of a gratin dish, a pie plate, or a glass platter (about 9 inches across and 1 inch deep) that is nice enough to go the dining room.
Mix half the reserved orange juice with the Grand Marnier and sprinkle it on the cake slices. Spread the mascarpone on top and arrange the orange segments on top of the mascarpone. Combine the remaining juice with the orange marmalade and coat the orange segments with it. Decorate with the mint.
Serve, using a large spoon to lift up the cake pieces.