Melt the butter in a saucepan, sprinkle with powdered sugar and let it melt. Stir until you get a golden yellow caramel. Deglaze the caramel with orange juice and dissolve the caramel in it. Add the kumquats and simmer gently for about 4 to 5 minutes. Finally, season with Grand Marnier to taste.
Kumquat compote goes well with parfaits or soufflés.