Having worked a lifetime ago at a Baskin Robbins, I'd be happy to tell you how it's done.

Take frozen cake layers (Whatever flavor you want) out of the freezer. Place one layer on a platform of your choice.

In a plastic (or other microwavable bowl) place 1/2 gallon of ice cream. Microwave on high for about 30 seconds. (It should have the consistancy of thick frosting.) Mix well with a spoon and spread evenly over frozen cake bottom. Freeze solid.

At this point some of their cakes have a fudge layer... or another ice cream layer. If this is what you want, place the cake with the initial layer of ice cream back in the freezer and freeze soild again. Then add the next layer of ice cream or fudge. (If using fudge, make sure it's cold)

Place remaining layer of cake on top and freeze again.

Take vanilla ice cream and heat in the microwave for the "frosting" on the cake. Spread evenly and freeze again. Decorate as you wish.

My favorite combo is peppermint ice cream with chocolate cake and fudge for the frosting. They roll this one up like a jellyroll and slice it diagonally in half and stand it up to look like two candles at Christmas time. MMM is it ever good.