There are two main ways to thicken this cake filling. Keep in mind that the filling will thicken more as it cooks, and again as it cools down.  Cook it down. You can continue to simmer the filling until the water cooks out of it, which will thicken it quite a bit. Just increase the cook time until it looks right.  Add more cornstarch. The best way is to add the cornstarch right at the beginning. But if you find that it's still way too thin (and you've given it a fighting chance to thicken first), you can add an additional tablespoon cornstarch. Just make a cornstarch mixture with a tablespoon of cornstarch and 1/4 cup of cold water. Mix until there's no lumps,  then stir the cornstarch slurry into the filling. Cornstarch works as it heats up, so avoid using hot water for this.