Zest a large lemon. Then juice the lemon; set the juice aside.
Add the vegan butter and lemon zest to the bowl of a stand mixer. Mix on medium to medium-high speed until thoroughly combined.
Add 4 cups of confectioner's sugar to the bowl, and start the mixer on low to combine the sugar & butter.
With the mixer still running, add the lemon juice a tablespoon at a time until you reach a spreadable consistency. (I increase the speed of the mixer to medium or medium-high once the powdered sugar is mixed enough that it won't fly out of the bowl).
Taste the frosting- if it's too tart (or if the frosting is too soft of a texture), then add in the last remaining ½ cup of confectioner's sugar, and beat at medium-high speed until smooth.
Optional: At this point, I change from the paddle attachment to the whisk attachment on my Kitchen Aid mixer. I turn the mixer on to high & beat the frosting for 2-3 minutes to help whip air into the frosting. IMO, it makes for a lighter & less sweet frosting.