Begin by placing your softened unsalted butter into a mixer bowl. Beat the butter on high speed for about 6-9 minutes, until it turns creamy and pale.

Gradually sprinkle in the confectioner's sugar while mixing on low speed, then increase the speed to high and whip for 3-4 minutes until fluffy.

Add the lemon extract, citric acid, and lemon zest. Mix on medium speed until well combined, scraping down the sides as needed.

Check the consistency; if too thick, add milk one tablespoon at a time while mixing on low until smooth and spreadable.

Give the frosting a final whip on high speed for 1-2 minutes for extra fluffiness.

If not using immediately, transfer to an airtight container and refrigerate for up to one week.