Cheese blintzes are a traditional Jewish food, always popular, but commonly served during the Jewish holiday of Shavuot. As a result, they are also completely kosher. To keep your cheese blintzes kosher, make sure to use kosher dairy ingredients like cheese, butter, etc. If you eat kosher, you likely already have certified ingredients in your pantry and fridge, but if not just look for the kosher symbol when you're out at the grocery store. It's a simple letter "K" or "U" with a circle around it, and will often be found in the lower right corner of the packaging, but it could be anywhere, so you may have to look around for a minute to spot it.