Line the bottom of a 9-inch springform pan with parchment paper.
Pulse the Fruity Pebbles for the crust in a food processor until mostly fine crumbs form.
Combine the cereal crumbs with melted butter until evenly coated.
Press the mixture firmly into the bottom of the prepared pan.
Freeze the crust for 15 to 20 minutes, until firm
Allow the vanilla ice cream to soften slightly. It should be creamy and spreadable but not melted.
Spread the softened ice cream evenly over the chilled crust.
Sprinkle 1 cup of Fruity Pebbles over the ice cream and gently press them into the surface.
Cover the pan tightly and freeze for at least 4 to 6 hours, preferably overnight.
When the cake is almost ready, beat the cold heavy cream, powdered sugar, and vanilla until smooth and fluffy with medium-to-stiff peaks.
Remove the frozen cake from the springform pan and spread the whipped cream over the top.
Decorate with the remaining Fruity Pebbles and optional sprinkles.
Freeze for another 30 minutes to 1 hour.
Let the cake rest at room temperature for 5 to 10 minutes before slicing.
Cut with a sharp, warm knife and serve immediately.
