Soft the unsalted butter at room temperature until slightly soft to touch. Butter should not be melted, runny or too soft. Place the butter into a mixer bowl and whisk on high speed for 6 to 9 minutes, until the butter is very fluffy, creamy and a pale creamy color. Scrape down the sides of the mixing bowl often to get all the butter whipped.
To the whipped butter, add the confectioner's sugar and continue whisking. Whisk first on low speed until the sugar is incorporated, then whisk on high speed for 3 to 4 minutes until the frosting is fluffy.
Next, add the lemon extract, citric acid, lemon zest and yellow food coloring. Whisk the flavorings into the buttercream for about 30 seconds, scrape down the sides of the mixing bowl and whisk again for another 30 seconds, until the flavorings and color is well incorporated.
This recipe makes enough frosting for a three layer, 8-inch cake; two-layer, 9-inch cake; or 24 cupcakes. This frosting will also work well for filling macarons or pastries. Buttercream can stand in the refrigerator for 1 week; cover well.