1. Make the Lemon Curd: Start by making the lemon curd. In a small saucepan, whisk together the egg yolks, granulated sugar, lemon zest, and lemon juice. Cook over medium-low heat, stirring constantly, until the mixture thickens. This should take around 5-7 minutes. Once the curd has reached a pudding-like consistency, remove it from the heat and stir in the cold butter until smooth. Allow the curd to cool completely before using it in the frosting.

2. Prepare the Frosting Base: In a large bowl, beat the softened unsalted butter until creamy and smooth. Gradually add the powdered sugar, beating on high speed until the frosting is light and fluffy, about 3-5 minutes.

3. Flavor the Frosting: Once the lemon curd has cooled, fold it into the whipped butter frosting, along with the lemon zest, cream (or milk), and a pinch of salt if desired. Beat until the ingredients are fully incorporated and the frosting is silky and spreadable.

4. Use or Chill: The Lemon Curd Frosting is now ready to use! Spread it on your favorite cakes and cupcakes, or refrigerate it until you’re ready to use it. Just be sure to give it a quick stir before frosting to ensure the perfect consistency.