Do I use pure glucose or sugar syrup for making a caramel filling for a cookie? I am trying to perfect my Dutch syrup waffles (stroopwafels). The cookies (waffles  are perfect but the caramel filling is giving me problems. I tried making the caramel filling from sugar, sugar syrup, corn syrup, and pure glucose but all of these make the cookies sloppy after a day. What is the best sugar for making the caramel filling? What is it what I do wrong?
It is possible that your syrup is not cooked down far enough, and the moisture content is softening the waffle cookies. It helps to batch-make syrups, using extended cooking times to ensure a thicker, less moist syrup, and just store what you don't use for next time (saves time and effort). You can also use gums or starches to provide extra thickening (modified corn starch is my shortcut of choice). Also, there is another cheat that works wonderfully: after the waffles are baked, brush the filling side of each one with undiluted egg white and give them all a few more minutes in the oven. That will create a shiny glaze that will help prevent moisture-leaching.