In this case - the turbinado is not fine enough to form nice cubes (doesn't pack well), so you need to add the brown sugar to essentially "fill in the gaps" and keep the cube together when moistened. Otherwise you don't have enough contact points for the sugar to hold together against itself.
Mix in the same was as above and form. Allow to dry on the wax paper.
Fun Options to personalize your sugar cubes:
- Use food coloring to color your sugar
- Store new (or used) vanilla bean pods in the sugar. The beans will add a lovely vanilla flavor to your sugar.
- Add edible glitter (found in speciality cake shops) to your cubes
Place them in a fun jar, add a ribbon, attach some tongs, and share with a friend!