Zest the oranges using a zester or vegetable peeler. Using a mortar and pestle, food processor or knife and chopping board combine the the zest and sugar until the peel is ultrafine.
Add orange sugar to a mixing bowl with the butter and beat with an electric mixer to combine.
Cut oranges in half and squeeze juices. Strain juice and add ⅔ cup juice and liqueur (if using) to the butter and whip together with the mixer.
Store in a jar or other container in the refrigerator until ready to use.