Start by whisking together the dairy-free milk and acv together and set them aside for a few minutes. This will be your vegan buttermilk.
In a large bowl, add all of the wet ingredients, followed by the dry ingredients, and whisk together.
Note: I highly recommend sifting the flour and cocoa powder, or you might get uneven clumps.
Split the cake into two even-sized, parchment-lined springform pans (mine are 6").
Bake the cakes at 350F for 35-40 minutes, until a toothpick comes out clean from the center. Baking times may vary depending on the kind of pan you use and your oven, so check up on your cakes!
While the cake is baking, whisk together the buttercream ingredients. I recommend using a stand mixer or hand mixer to beat the butter first, then slowly adding the powdered sugar, vanilla, salt, and milk while mixing on medium speed.
Set aside about ½ cup of frosting. Add in a few drops of red dye to the large frosting batch until you get a light pink color. Add a few drops of green to the smaller batch.
Once the cake has fully cooled, level the cake, add a layer of pink frosting on top of one of the cakes, then place the other on top.
"Crumb coat" the cakes by covering them in a layer of frosting. Refrigerate until that layer hardens. Then, add the rest of the pink frosting to the cake.
Place the green frosting in a piping bag with a small tip and pipe the letters "Happee Birthdae Harry". I recommend outlining the letters lightly with a toothpick first, then piping the frosting on top of your outline.
I also used a toothpick to carve out the dent in the cake from Hagrid sitting on it, just like in the movie.
Slice your cake and enjoy it with your Harry Potter marathon!