Whip the butter.
Place the softened butter in a stand mixer fitted with the whisk attachment. Beat on medium-high speed for about 5 minutes, until pale, light, and fluffy.
Scrape down the sides and bottom of the bowl as needed.
Add the flavoring.
Add the vanilla and salt. Mix for 1 minute.
Gradually add the sugar and cream.
Reduce the mixer speed to low. Add the powdered sugar in several batches, alternating with the heavy cream. Start with 2 tablespoons cream and add more as needed.
Mix until combined, scraping down the bowl between additions.
Whip until fluffy.
Increase the speed to medium-high and beat for another 2–3 minutes, until smooth and fluffy.
Adjust the consistency.
For softer frosting, add more cream 1 teaspoon at a time. For firmer frosting, add a little powdered sugar at a time.
Use to frost a completely cooled cake or transfer to a piping bag for decorating.