In a large mixing bowl with an electric hand mixer or the bowl of a stand mixer, add in room temperature unsalted butter. Whip the butter for 5 minutes on medium-high speed, scraping down the sides occasionally. The butter will be light, fluffy, and pale in color.
Add one half of the powdered sugar and mix on the lowest speed setting until fully combined. Add remaining powdered sugar and continue to mix on low until incorporated.
Add vanilla extract, mint extract, and salt and mix on low until combined.
Increase speed to medium on the mixer and slowly drizzle in the heavy cream. Scrape the sides of the bowl occasionally to ensure the frosting is well mixed. Add a few drops of green gel food coloring, if desired.
Turn the speed on the mixer to medium speed and whip for an additional 3-5 minutes until the buttercream is light and fluffy.
If desired, switch to a paddle attachment on the stand mixer and mix buttercream on low speed for 2 additional minutes to remove air bubbles to create smooth frosting.