Finely chop the chocolate and set it aside. Add the non-dairy milk to a saucepan and place it on medium heat until gently simmering, but do not boil.
Remove the saucepan from the heat and add the chopped chocolate, vanilla extract, and sea salt and stir with a rubber spatula until smooth. Set the ganache mixture aside for 15-20 minutes to cool to room temperature.
Add the vegan butter to a large mixing bowl (or stand mixer if using) and whisk with an electric mixer for 5 minutes, until light and fluffy.
Sift in the cocoa powder and whisk again for 1-2 minutes to combine. Then sift in the powdered sugar and whisk for another 2 minutes until it has been incorporated into the frosting.
Slowly add the cooled chocolate ganache to the buttercream in 3-4 increments, whisking to combine it into the buttercream each time. Once you have added all of the ganache, continue to whisk the buttercream for another 2-3 minutes until light and fluffy.
For best results, use the buttercream immediately to frost your vegan cake or cupcakes. You can store this buttercream in an airtight container in the fridge. It will harden when chilled, so before use, allow it to come to room temperature until it comes back to a creamy consistency.
Vegan butter: Take this out of the fridge to sit at room temperature for 1-2 hours. Make sure you use vegan block butter, otherwise known as butter sticks. Spreadable butter from a tub or margarine will throw off the recipe due to its higher water content. I use flora plant butter which is nut-free, coconut-free, and gluten-free.
Powdered sugar: Some US-made white sugar is not vegan, check the brand or use organic to ensure it is vegan-friendly. This recipe needs powdered sugar and will not work with a liquid sweetener. You can use powdered coconut sugar, but it must be blended down to powder in your blender/food processor first.
Make sure the ganache is at room temperature before adding it to the vegan chocolate buttercream. Adding it while warm may cause your frosting to split.
For frosting layer cakes: This recipe will generously frost a 2 layer 8-inch or 9-inch cake, or a triple layer 6-inch cake. For an 8-9-inch sheet cake, I would cut the recipe by one quarter.
For frosting cupcakes: The recipe yields enough buttercream to generously frost 12 cupcakes, or lightly frost 24 cupcakes.