In a small, non-reactive saucepan (see note), whisk together egg yolks, lemon juice, sugar, and salt.

Add butter and cook over medium-low heat, whisking constantly until mixture is thickened (the whisk should leave trails through the curd and it should be able to coat the back of a spoon, see video or photos in post for visual if desired).

Remove from heat and pour through a fine mesh strainer into a heatproof bowl. Allow to cool completely before proceeding (you can transfer to the refrigerator to cool faster).

Using an electric hand mixer or stand mixer, beat 1 cup softened butter until light and fluffy (high speed about 30-60 seconds). Gradually add powdered sugar until completely combined.

Drizzle in cooled lemon curd and stir until completely combined. Be sure to scrape the sides and bottom of the bowl to ensure ingredients are well-combined. Note: if frosting is too thin you can add additional powdered sugar, up to 1 cup/125g.

Spread or pipe over cake or cupcakes.