In a medium saucepan, whisk together the sugar, cornstarch, and salt. Add the milk, water, and lemon juice and whisk until well combined.

Place the pan over medium- low heat and cook, stirring constantly, until thickened. This could take 10- 20 minutes.

Strain the hot lemon curd into a bowl and mix in the butter and gel color (if using). Stir until the butter is completely melted.

Press cling wrap onto the surface of the curd and let it sit on the counter until cooled down (1- 2 hours). Cover the bowl and place the curd in the fridge over night.

Beat the butter until pale and fluffy, about 8- 10 minutes. Scrape the bowl down.

Add the powdered sugar in 3- 4 additions. After each addition, beat on low to combine, then bump the speed up to medium and beat for a minute, or until smooth. Scrape the bowl down.

Warm the chilled lemon curd in the microwave slightly so that it is nearly room temperature. Add a tablespoon of lemon curd at a time to the buttercream and beat to combine. Continue adding curd until the frosting is at the right consistency or flavor. I used about 8 tablespoons total. Taste and adjust flavors as needed. If desired, add gel food coloring to create a pretty yellow.