Homemade ice cream
What do i need to add to keep my homemade ice cream from getting rock hard when putting the leftovers in the freezer?
There are plenty of things that can help with that, but instead of adding them simply to make the ice cream scoopable immediately after taking it out of the freezer, what about simply microwaving it? This is my favorite method to make ice cream softer very quickly. If it's a container that won't fit or can't go in the microwave, simply put it in the refrigerator about 10 - 20 minutes (depending on the size of the container and temperature the ice cream was before it went into the refrigerator) before you're ready.
Here are a few things that will make your ice cream softer by either limiting the size of ice crystals or lowering the freezing point of the base:
Locust bean gum
Store your ice cream in a shallower container to increase the surface area-to-mass ratio so that, when microwaved or put in the refrigerator, it rises in temperature more evenly and faster.
Also, always store your ice cream with plastic wrap pressed down right onto the entire surface of the ice cream to prevent freezer burn, off flavors, and ice crystals from building up on the surface of the ice cream.
It's good that you're having this issue because that means that, more than likely, your freezer temperature is as low as it should be. Whatever you do, don't do what most of us do to solve your problem which is turn up the temperature on the freezer.
In addition to the pointers that 1POINT21GW suggested, also consider "resting" your ice cream before eating and/or serving.
Take the ice cream out of the freezer and let it simply rest, or sit, on your kitchen counter for about 15 minutes before scooping and serving (or eating).
Ice cream -- homemade or store bought -- benefit from resting. Few things taste good frozen solid (except for maybe flavored ice cubes).
ipsedixit is right, the higher the temperature of the ice cream the better. Not only does a higher serving temperature make for easier scooping, it also makes for a better tasting product. Our taste and smell senses work better with food that is warmer rather than frozen solid.
Look to serve ice cream around 5 - 10 degrees F. To do this you can bring it up to temp by either refrigerating it, leaving it out at room temperature as ipsedixit suggested, or, to speed things up a bit, microwaving it. My favorite is the microwave method because it's faster and it warms the ice cream more evenly since it heats the water molecules at the very center of the ice cream mass from the very beginning of the process rather than from the outside in like the other two methods, thus, potentially, leaving unevenly temperatured ice cream and, possibly, even some melted on the outside - not that there's anything wrong with eating melted ice cream.