Wash and clean the raspberries and let them drain. Zest and juice a lemon then set these aside.
In a 2-quart saucepan, add the raspberries, sugar, and lemon zest. Heat over medium-high heat until it comes to a boil. Turn the heat back to medium (you want a low boil) and start to break up the raspberries by smashing them against the sides of the pan. I use a wooden spoon since it has a broad flat part on the back of the spoon.
Continue stirring and breaking up the raspberries for about 10 minutes. Next, mix the cornstarch and lemon juice in a small cup to get the lumps out. Next, stir it into the raspberries so the jam thickens on a low boil. Continue for about 5-7 minutes, then take it off the heat. Leave it in the pan for about 10 minutes.
Finally, pour the jam into a glass canning jar with a seal on the lid. I let it cool down for about 30 minutes before I seal it with the top and put it into the refrigerator. I like to let it sit overnight in the fridge before using it.
Notes
If you are going to use this for thumbprint jam cookies, I would at least double the cornstarch and lemon juice for added thickness. This is a thinner consistency, so you can spread it throughout a cheesecake.