If you want to freeze these cookies for later, I recommend freezing the dough for later as opposed to freezing already baked thumbprint cookies with jam. Make the cookie dough as normal and form into balls, then create the indentations in the cookies. Then, freeze them for up to one month. When you are ready to bake, simply place the already formed cookie dough onto baking sheets lined with parchment paper, add the jam, and bake. You might need to add 1-2 minutes to the baking time when baking from frozen.