5 Syrup
This is a simple old classic, but it works really well. Just drizzle some syrup over the top of your shortbread and you’ll have a sticky, sweet treat. You can get syrup in a number of different flavors of course, and strawberry syrup works particularly well in this scenario.
Another option is to use the syrup as mentioned before: as an adhesive. Pour some syrup over the top of your shortbread, and then dip that shortbread into the sprinkles. Or why not cover the shortbread in the syrup and then just sprinkle on top?