Allow butter to come to room temperature.
While butter is softening, sift powdered sugar so there are no large clumps.
Add whisk attachment to the stand mixer as well as unsalted butter.
Whisk butter on low-medium speed until it begins to cream.
Next, returning the mixer to the lowest speed, add powdered sugar 1/2 to 1 cup at a time. Continue mixing until all ingredients are incorporated.
Finally, add vanilla & half of the whipping cream. Whisk for 2-3 minutes (add in additional whipping cream if needed) until a thick buttercream icing is formed.
For frosting a cake: 1) leave at room temperature for best spreading consistency & 2) if adding color, place 1/3 of icing you want to leave plain into a separate bowl before mixing color into the rest of icing.