Soften the butter at room temperature. In the bowl of a stand mixer or large bowl, whip the butter until fluffy using the whisk attachment.
Add the powdered sugar, half at a time, and mix on low to medium speed until combined.
Whisk in the lemon zest, extract, and salt. Keep the mixer on low speed and pour in the heavy cream. Scrape down the sides of the bowl as needed.
Add more cream until the desired consistency is reached. Whip on high speed until the buttercream is light and fluffy.
Use the buttercream to decorate a small cake, your favorite lemon dessert, or a delicious strawberry cake.