Beat the Butter - In the bowl of a stand mixer fitted with the whisk attachment (or handheld electric mixer), beat the butter on high speed for 5 minutes or until lightened in color and whipped, scraping down the bowl as needed.
Sift the Powdered Sugar - Meanwhile, sift 4 ½ to 5 cups of powdered sugar. Reduce the mixer to medium-low speed and add powdered sugar one heaping Tablespoon at a time, letting it incorporate with each addition. Add powdered sugar to taste and beat until it is fully incorporated, scraping the sides of the bowl as needed.
Mix In - Add ¼ tsp fine salt (or to taste), then add 4 tsp Vanilla Extract, beating until incorporated.
Thin with Heavy Cream - Add 3 Tbsp heavy cream or milk, 1 Tbsp at a time, and add it to the desired consistency. Beat another minute until whipped and well combined.
