Grease a nine-inch springform pan with cooking spray and set aside.
Melt 3/4 cup of the butter in a large saucepan over medium heat. Reduce the heat to low and add eight cups of marshmallows, stirring frequently until melted.
Remove from the heat and stir in one teaspoon of vanilla and salt. Add the cereal and remaining marshmallows and stir until well combined. Scatter over 1/4 cup of the sprinkles, stir just until combined.
Using a rubber spatula sprayed with a little cooking spray, scrape the mixture into the prepared pan. Lightly press into an even layer, being careful not to compress the mixture too much. Scatter the top with another tablespoon of sprinkles and set aside to cool to room temperature.
Make the Frosting
Meanwhile, make the vanilla buttercream by beating the remaining 1/2 cup of butter with two cups of icing sugar until light and fluffy.
Beat in the remaining 1/2 teaspoon of vanilla extract and the cream and season with a small pinch of salt. If the buttercream looks a little thin, add in some more icing sugar.
Colour the buttercream with a few drops of food colouring, if desired and transfer to a piping bag or resealable bag fitted with a decorative tip.
Set aside for assembly.
Assemble the Cake
Remove the cake from the springform pan and place it on a cake pedestal or serving plate.
Pipe the buttercream around the edge and top of the cake and scatter with more sprinkles. Serve and enjoy!
TIP: Store the cake covered at room temperature for up to 4 days.