Place cereal in a large bowl.
Grate or shave 2 tablespoons of chocolate from candy bar; set aside.
In a heavy saucepan, melt butter and remaining chocolate.
Pour over cereal and stir to coat.
Press into the bottom of a greased 10-in.springform pan.
Freeze for 30 minutes.
Spoon ice cream over crust.
Freeze for 15 minutes.
Spread with whipped topping; sprinkle with the shaved chocolate.
Cover and freeze for several hours or overnight.
Top with candy.
Remove cake from freezer 5-10 minutes before serving.
Remove sides of pan; cut with a sharp knife and serve immediately.