In a stand mixer or with a hand held mixer beat the sugar and orange zest for 2 minutes to release the orange oils.
Add butter to the bowl and cream. until pale yellow.
Add eggs one at a time beating well in between each addition. Beat until a pale yellow, about 3 minutes.
Turn mixer on low and slowly add orange juice or lemon juice.
Pour mixture into a heavy sauce pan and heat on medium-high, whisking constantly until thickened and has reached 175 F on a candy thermometer. Be sure to continually whisk to avoid separation.
Once thickened to remove any lumps strain through a fine mesh strainer into a bowl.