Wash and cut the mirabelle plums, discarding any that are not fully ripe or are damaged.
Pour the mirabelle plums into a glass jar. Add the rock sugar on top, then pour in the schnapps (fruit brandy) or gin. Place the jar in a cool, dark place and shake it gently every 1-2 weeks for 8 weeks.
After 8 weeks, strain the liqueur through a coffee filter lined with filter paper into bottles. Store the bottles in a cool, dark place for 4-6 weeks before serving.