Place the butter, cream cheese, lemon juice, and vanilla in a large mixing bowl, and beat on medium-low speed until smooth.
Scrape the bottom and sides of the bowl with a silicone spatula, and add about half the powdered sugar.
Stir in the powdered sugar until combined, then scrape the bottom and sides of the bowl again, and add the remaining powdered sugar and the salt.
Stir together until combined, then turn the mixer up to medium-high speed and whip until the frosting is fluffy and stiff. (Be careful not to over-whip the cream cheese frosting, or it could become runny. As soon as the frosting looks stiff, turn off the mixer and use the frosting immediately.)
Video
https://youtube.com/watch?v=sxsnDyQJkBM
Notes
Makes enough to frost 24 cupcakes, 1 triple-layer, 6-inch diameter cake, or 1 double-layer, 8-inch diameter cake.