There are a number of nation-wide chain restaurants that have been making the "mix-in" style ice cream desserts for years. Nothing, however, compares to the old fashioned style of ice cream dessert made by hand mixing in the ingredients into the ice cream. With a little preparation, you can start making these desserts the old fashioned way, as well.
Freeze the marble. The first, and most important part, of putting together an ice cream mix-in is to have the marble slab you are working with at a very low temperature. Marble is used because it holds its low temperature for a long time and doesn't need to be refrozen between each and every mix-in. Clear out your freezer, and let the marble freeze in there overnight. Do not remove it until just before you want to use it for the mix-ins. Take it out at the last minute. Once out, set it up in an area where there is a good deal of room for moving around. Making a mix-in can get physical.
Choose the right kind of ice cream. High quality ice cream works the best for mix-ins. The higher the quality ice cream, generally speaking, the higher the amount of butterfat. The more butterfat that is in the ice cream, the more pliable it will be while mixing, and the longer it will retain its temperature while still remaining creamy. Check the sides of the containers you buy to inspect the amount of butterfat before buying. Sometimes the pricier brands will be lower in fat, and with a mix-in, you don't want that.
Pick candy mixers that will work. Not all candies and candy bars are great choices for a mix-in. The ones that work the best are those that can break up easily into smaller pieces. Peanut butter cups, chocolate sandwich cookies and chocolate-covered peanut butter bars work very well. Gummy candies don't work well, because they don't break up easily, and they harden as they become cold. Also, think about the flavor combinations. If you are using strawberry ice cream, then peanut butter candy may not be the best mixer.
Roll up your sleeves and get physical. After you have let the ice cream sit in room temperature for five minutes, retrieve the slab from the freezer. Place it in your prechosen spot and open the ice cream. Take two large paddles-full, and mix together on the marble. Mix a little first without any mixers to get all of the ice cream at the same temperature. Create a small well in the middle of the pile of ice cream and place the mixers in there. Less is more with this. Now, mix the ice cream from the outside in, folding the ice cream on top of the pile of mixers, repeating and turning the piles of ice cream as you mix. It should take about three minutes or so to get the mixers completely mixed through. Once mixed, form into a ball and serve in either a dish, or large waffle cone, and enjoy!