Start by placing your softened unsalted butter into a large mixing bowl. Using an electric mixer, beat the butter on medium speed for about 2-3 minutes until it becomes light and fluffy.
Gradually add in the powdered sugar, lemon juice, and lemon extract. Continue mixing on low speed for about 3-4 minutes until well combined.
Add the sweetened condensed milk to your mixture in tablespoon increments, mixing well after each addition for about 5-7 minutes.
If the buttercream feels too thick, add a splash more lemon juice or an extra tablespoon of sweetened condensed milk. If it’s too runny, gradually insert more powdered sugar.
Stir in the lemon zest at this stage, mixing it evenly into the frosting.
Your Lemon Sweetened Condensed Milk Buttercream is now ready for use! Spread over your cooled cakes or cookies.