Add the egg yolks, sugar, juice, and zest to a saucepan and whisk to combine.

Place the pan over medium heat and bring to a boil, whisking gently. Once it starts boiling, the orange curd will begin to thicken. Let it cook for 5 minutes and add the butter during the last minute. Once it has cooked for 5 minutes and the butter has melted, remove from heat.

Pour the hot orange curd into a heat-resistant bowl or jar and let it cool completely. Once cooled, store it in the fridge for up to two weeks.