Cream vegan butter in the bowl of your stand mixer fitted with the paddle attachment.
Meanwhile, measure 4 cups of powdered sugar into a sifter and sift.
Add all of the powdered sugar into the mixer and with the speed on LOW, mix for about 1 minute. Scrape the bowl with a spatula to be sure everything is incorporated.
Add vanilla into the mixer and stir to incorporate. Scrape the bowl again.
With the mixer on medium speed, slowly stream the molasses into the bowl.
Mix the buttercream on medium speed until its light and fluffy- about 2 minutes- stopping to scrape the bowl down a few times.
Fill cookies, frost a cake, or eat with a spoon. This buttercream will keep in your fridge for up to a week and can be frozen for up to a month!