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.
2 large oranges, ¼ cup granulated sugar
Add orange sugar to a mixing bowl with the butter and beat with an electric mixer to combine.
½ pound unsalted butter
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.
3 tablespoons orange liqueur
Store in a jar or other container in the refrigerator until ready to use.