Beat butter in electric mixer on high speed until light and very fluffy (4-5 minutes). Beat in one-third of icing sugar and half crème de menthe, beat well to combine (2-3 minutes). Add half remaining icing sugar and remaining crème de menthe, beat well to combine (2-3 minutes), then beat in remaining icing sugar(2-3 minutes). Refrigerate until mixture is a workable consistency (30-40 minutes). Form heaped teaspoons of mixture into mounds, place on a tray lined with baking paper, freeze until firm but not frozen (10-15 minutes).
Meanwhile, melt chocolate in a heatproof bowl placed over a saucepan of simmering water. Dip creamed peppermint mounds one at a time into chocolate, shaking off excess, then roll in cocoa. Place on a tray lined with baking paper and refrigerate until firm (30-40 minutes). Store refrigerated in airtight containers.