Preheat oven to 350F/180C. Line two 8-inch pans with parchment paper.
Combine the cake mix and instant pistachio pudding mix in a large bowl.
Now add the water, buttermilk, and mashed avocado.
Beat all the ingredients well, using a handheld electric beat or a wooden spoon.
Use a one cup measuring cup to evenly measure out the cake batter into the prepared two pans.
Tap the pans to break air bubbles if any.
Bake for 32-36 minutes or until an inserted toothpick comes out clean.
While the cake is baking, you can start making the frosting.
For Frosting:
Before even setting out to make the frosting, put a large bowl and the beaters in the freezer. This step ensures to get a good whipped cream frosting.
Take the chilled bowl from the freezer and pour the whipping cream, powdered sugar, pistachio pudding mix, and salt.
Now whip until you get stiff peaks about 3-5 minutes on the maximum setting. Some blenders have an option for whipping cream. The frosting should be thick and spreadable.
For Assembling the cake:
If there are domes on the cakes, level it by slicing it off the tops.
Put the bottom layer of cake on a cake sheet if using or on the plate, you wish to serve it. Add about 1 cup of the frosting mixture on top of the cake and spread until even.
Put the top layer of cake on, and apply the frosting to the top and sides of the cake.
Notes
Baking time might be slightly higher for cakes which have pureed fruit like unsweetened applesauce, mashed banana or in this case mashed avocado. So test for doneness using a toothpick. Just don't remove the pan from the oven once the specified baking time in the recipe is reached.
I used two 9-inch round cake pans to make this layered pistachio cake recipe. Only after the cake was baked, I realized that I should have used 8-inch pans instead because the layers were thin and one of the cake slightly cracked which I think was due to the lack of thickness of the cake.
The original recipe had mentioned using 1 cup whipping cream to make the pistachio frosting, but I felt that the quantity of icing won't be enough to frost the entire cake. So I added another cup of whipping cream and made enough frosting. This quantity was just perfect to frost the whole cake.