Soften vegan butter until it reaches near room temperature.
Whip butter with an electric mixer using the whisk attachment for 1 minute.
Add powdered sugar 1/2 cup at a time whipping for about 30 seconds between each scoop.
Add 1 tsp lemon extract and the zest of one lemon then continue to whip for another minute until light and airy.
If needed, slowly add a small splash or two of dairy-free milk while whipping the buttercream until the desired consistency is achieved.
Video
Notes
Whip your frosting for a long time, at least 5 minutes total. Whipping the sugars and fats together will cause them to blend better and whip in air pockets that will make your buttercream light and fluffy.
Only add plant milk if needed. You want your frosting to be smooth and creamy, yet firm enough to pipe perfectly.
If you struggle to make your piping look nice, I have found that this cupcake decorating tool is easier than piping bags for beginners. (I have no cake-decorating abilities at all, but it helps my cupcakes look so much better than when I use a piping bag).