First of all, you need a pan big enough to hold 1/2 kg cake plus 300 ml ice cream. I have taken 6" round cake pan with a loose bottom. You can also opt for Cheesecake pans. Or simply, any pan and lay cling film in it, large enough to hang outside the pan also.
Mix powdered sugar in water and keep it aside.
Take out both the ice creams from the freezer and take the required quantities in bowls and let it be on the kitchen counter for 5 to 10 minutes. In the meantime do the following preparations. You need soft ice cream and not melted one. So, in case it becomes too loose then keep it back in the freezer.
Now, divide the cake horizontally into 3 parts so that you have 3 equal discs.
Lay one layer in the pan. Put 1 or 2 tablespoon sugar syrup on it to moisten it.
Now, put half of Chocolate ice cream on it and spread evenly.
Keep the second layer, repeat the process but this time put butterscotch ice cream. Use all the Butterscotch ice cream.
Keep the last layer and repeat the process ending with chocolate ice cream. Ensure that it slides on to the sides of the pan also.
Freeze it for 4 to 6 hours or maybe overnight.
Take out, run a knife around the edges and slide the cake upwards.
In the process, if the ice cream starts melting then keep it back in the freezer.
Take out, decorate and serve.