In a ceramic saucepan, mix the granulated sugar and orange zest together, massaging the zest into the sugar for about 2 minutes.

Once fully combined, whisk in the egg yolks (or whole eggs) until smooth, then blend in the orange juice and fine salt.

Place the saucepan on medium-low heat and cook the mixture, stirring constantly for about 20-25 minutes.

Once thickened, remove from heat and add the cold, unsalted butter; stir until completely melted.

Pour the mixture through a fine mesh strainer into a clean bowl, discarding any solid bits.

Cover with plastic wrap touching the surface to prevent a skin from forming; cool to room temperature before refrigerating.