In a small saucepan, add the 2 cups of citrus juice. Heat over medium-high until it comes to a gentle boil. Once the citrus juice is boiling, remove the saucepan from the heat. Add the zest from 1 lemon to the hot citrus juice, stirring to incorporate.
Begin to slowly pour the hot citrus juice into your egg and sugar mixture while continuously whisking. After you’ve combined the citrus juice with the eggs and sugar, pour the mixture back into your saucepan.
Return the saucepan to the stove and heat the mixture over medium heat, stirring frequently. Use a kitchen thermometer to monitor the temperature. Once the mixture reaches 180°F (82°C) and has thickened to a custard-like consistency, remove from heat
Immediately pour the mixture through the strainer and into the prepared bowl. After straining, add the ½ cup of cubed unsalted butter and ¼ teaspoon of salt to the mixture. Stir well until the butter is fully melted and incorporated.
Once cooled, transfer the citrus curd to an airtight container and store in the refrigerator. The curd will continue to thicken as it cools.