For the cake layers:

1. Spray 3 8X1 ½ cake pans with cooking spray or lightly oil (if cake pans are not non-stick)
2. Melt the butter in an extra-large pot over medium heat. Add the marshmallows, and cook, stirring, until they melt completely, about 5 minutes. Remove the pot from the heat and stir in the vanilla and salt. 
3. Add the rice cereal and stir with a rubber spatula, scraping up from the bottom, until the cereal is completely coated. Divide the cereal evenly into the 3 prepared cake pans.
4. Place a piece of parchment paper on top and using a drinking glass, large spoon or your hands, pat it down into an even layer. Let sit at room temperature until firm, about 30 minutes.

For the buttercream icing:

1. Add vegan butter to a large bowl. Using a hand or stand mixer, cream until smooth. Add vanilla and salt. Slowly add the powdered sugar 1 cup at a time at low speed gradually increasing speed until thick and smooth.
2. If your consistency is too thick, add a little plant milk a drop at a time, and if it’s too thin, add 1/4 to 1/2 cup more powdered sugar.
3. Turn the “cake” onto a serving platter. Using a piping bag pipe one thick layer of buttercream on top.
4. Repeat with remaining layers of the “cake”. Decorate the top with a ring of buttercream roses and sprinkles, or to your liking.