Combine orange zest, juice, sugar, and eggs in a heatproof bowl.
Set the bowl over a pot of simmering (not boiling) water, ensuring the base doesn’t touch the water.
Whisk constantly over gentle heat until thickened (±10–15 min). The mixture should coat the back of a spoon.
Remove from heat. Strain through a sieve for silky smoothness, or leave the zest in for texture.
Once slightly cooled, blend in the softened butter with an immersion blender until glossy.
Spoon into a clean jar and refrigerate. Keeps for ±1 week.