Preheat the oven to 350ºF and line 3 cake forms (7 inches in diameter each) with parchment paper (or grease them).
To a mixing bowl, add the soy milk and apple cider vinegar. Mix to form the vegan buttermilk, then add the oil, vanilla, sugar, salt. Sift 1 1/2 cups of the flour (set the other 1/4 cup aside), then add it to the bowl with the baking powder, and baking soda. Mix again.
To a blender or food processor, add half of the pistachios and process until crumbly. Add the other half of the pistachios and process again until one part is very crumbly and the other half is chunky (resembling chopped pistachios). Set aside.
Divide the batter into 3 equal parts in 3 separate small bowls. To the first (this will be your top layer), add 2 1/2 tablespoons of all-purpose flour and 3 tablespoons of the pistachio mix. To the second (this will be your middle layer), add 1 1/2 tablespoons all-purpose flour, 4 tablespoons of the pistachio mix and about 5-10 drops of the green food dye. To the third (this will be your bottom layer), add 5 tablespoons of the pistachio mix and about 15 drops of the green food dye (plus more if needed.) Mix each of them until everything is well incorporated. The middle layer should be light green and the bottom layer dark green. If they’re too similar, add a little extra dye to the bottom layer.
Next, pour each layer into their own lined baking form and place in the oven (slightly below the middle). Bake for 25 minutes or until a toothpick comes out clean, then let cool for at least 30-35 minutes (Tip: to make sure the layers are cool enough to be frosted, you can wrap them each in plastic wrap and place in the freezer for a bit once cool enough to handle.)
Make the Frosting & Decorate the Cake
Using either a blender or a food processor, process the pistachios until you get a very fine, flour-like texture (much finer than the texture used for the batter). Sift them to make sure you retain only the finest part (you can set any remaining pistachio chunks aside and striking them between each layer when frosting the cake.)
Next, cut the vegan butter into cubes and add it to the mixing bowl of your stand mixer. Cream the butter, then add the powdered sugar and pistachio flour little by little. Next, add the cream cheese and vanilla extract and mix until you get a smooth, creamy texture.
Remove about 2/3 of the frosting and set aside. To the remaining part, add a few drops of green food dye (to achieve a very light green) and mix them in.
Once the cake layers are cool enough (ideally fully cooled), layer them, spreading the light green frosting in between each layer and then a thin layer all around the cake. If you have any light green frosting left over, set is aside for when you’re piping the medium green layer on the cake.
To create the ombre buttercream roses, start with the lightest (top) part. Add some of the frosting (without food dye) to a piping bag with a nozzle (such as a 2D nozzle) and cover the top of the cake (see the recipe video for piping instructions).
If you had any light green frosting left, you can mix it with the rest of the frosting using the stand mixer. If it’s too light, add a little extra green good dye. Add some of it to a piping bag and repeat the process until the middle of the cake is covered and there is just enough space for one last layer at the bottom (note: if you’re using a smaller nozzle, you’ll be able to add two layers per shade of green or add more more shades).
Add more food dye to the rest of the frosting until you get your desired shade of dark green, then repeat the piping process until the cake is fully decorated.
Store leftovers in an airtight container in the fridge for up to 3-4 days.
The prep time includes the time to make the frosting and decorate the cake.
The measurements listed in the instructions refer to the default serving size (8 slices.) While changing the serving size automatically adjusts the ingredient list, it won't change the instructions, so be sure to adjust the amounts accordingly.