Heat a double boiler (or a heat-proof mixing bowl) placed in a pot of water on the stove over medium heat.
Whisk together the sugar, eggs, egg yolks, orange zest, orange juice and lemon juice together in the top portion of the double boiler.
Cook, whisking often, until the mixture thickens and reaches a temperature of 170 degrees. Remove from the heat and immediately whisk in the butter.
Strain the orange curd through a fine mesh strainer into a glass jar or storage container. Allow to cool for 15-20 minutes then press a piece of plastic wrap across the top and refrigerator for at least 1-2 hours to firm up a bit. Store sealed in an airtight jar in the refrigerator.
Serve the orange curd on crepes, pancakes & waffles, toast, muffins, or ice cream. Enjoy.