Place the soft, room-temperature butter into a mixing bowl. Then, use an electric hand mixer to whip it for 2 minutes.
Next, add the sifted powdered sugar and continue whipping for another 2 minutes until the mixture is pale and fluffy.
Then add the full fat cream cheese and vanilla extract and continue whipping for 2 more minutes.
Now, very slowly, start adding the lemon juice while the mixer is running. You can stop whipping once the lemon juice is incorporated.
Transfer the frosting into a piping bag and chill it for 30 minutes before using.
The lemon cream cheese frosting can be stored refrigerated for 2-3 days.
Notes
The recipe above is enough to frost 12 cupcakes. To frost cakes, you will need approximately 2x for a 6-inch cake, 2.5x for a 7-inch cake, and 3x for an 8-inch cake (depending on how much frosting vs. sponge ratio you prefer on your cakes and whether you are using any other filling in between the layers).
Make sure you read my Expert Tips section above to maximize your success. A short recipe alone is not able to cover all the necessary details and science behind baking.
Measure your ingredients with a digital scale for accuracy.
The cream cheese needs to be high in fat. Using the low-fat version may result in a thinner frosting due to the excess water content.
Adding the lemon juice to the frosting needs to happen very slowly step-by-step or the frosting might break.
Because softened butter is being used, the frosting may still be on the softer side right after it's whipped. If this is the case, chill it for 30 minutes before piping. However, if it's chilled for too long, the butter will solidify, and you might find it difficult to pipe. If this happens, you need to let it soften a little before using it.