Place a mesh strainer on top of a medium-sized mixing bowl. Add the powdered sugar and cocoa powder into the strainer and sift together. Set aside.
Add the softened vegan butter to the bowl of a Stand Mixer(or use a large Stainless Steel Bowl with a Hand Mixer). Beat on medium speed until fluffy and pale for 2-4 minutes.
Now add the vanilla extract and mix on medium speed until incorporated (about 30-60 seconds)
Add 1 cup of the cocoa and powdered sugar mixture over low speed until it's mostly incorporated, scraping down sides as needed. Now turn up to medium-high speed and mix until smooth and fluffy. Repeat this step until the mixture is gone. Taste and add more sugar if needed.
On medium speed, add the milk 1 tablespoon at a time until desired consistency is reached.
Use right away or store in the fridge for a few weeks in an airtight container. The frosting will harden in the fridge. Just stir it and let it sit for 10 minutes before decorating your baked goods. You can also whip it again for a few minutes in the mixer to make it extra fluffy.
STAND MIXER: you can use the paddle or balloon whisk attachment. The whisk adds more air into the frosting which gives it a slightly whipped texture. If you want a more traditional buttercream, use the paddle attachment.
FROSTING: it can stay out at room temperature for quite a while without melting, so you can leave the decorated baked goods out. However, if it's too warm, the frosting will start to soften. I prefer to store the decorated cake/cupcakes in the fridge and take them out an hour before serving. This also helps set the frosting and prevents it from getting too soft.
If you'll be adding sprinkles or decorations on top of the frosting, make sure to do so right away or they won't stick.
If you're using regular cocoa powder for the frosting, you may want to add ¼ cup more to the recipe. Taste before adding more.
Use organic powder sugar to ensure that it's vegan.
This recipe makes enough for 12 cupcakes or a double layer 8/9-inch cake.