Make the filling: Combine the sugar with 2 tablespoons water in a medium saucepan (preferably with a light-colored bottom so you can see when the sugar colors) and stir with your finger to make a sandy mixture. Cook over medium heat, swirling the pan so the sugar cooks evenly and using a wet pastry brush to remove any granules of sugar from the sides of the pan (otherwise they’ll burn and make your sugar bitter), until the sugar syrup is a light caramel color, about 5 minutes.
Add the blackberries. When you do this, the sugar will immediately harden. Don’t be alarmed! Just keep cooking until the sugar melts, stirring as it becomes feasible to stir, until the blackberries begin to lose their shape and all the solid bits of sugar have melted, about 5 minutes. Turn off the heat.
Remove about ¼ cup of the syrup from the blackberries and put it in a small bowl. Add the cornstarch, lemon juice, vanilla and cinnamon and stir to dissolve any lumps. Add this mixture to the pot with the berries. Stir to combine and cook over medium-low heat for 2 to 3 minutes, until the liquid starts to thicken. Turn off the heat.
Combine the remaining berries in a large bowl. (If the strawberries are enormous, cut them in half or quarters.) Add the cooked blackberries, making sure to get all the syrupy goop out of the saucepan. Fold the raw berries into the goopy blackberries and refrigerate for about 1 hour (or as long as overnight) to chill. (You can put this in the freezer to speed up the process, but make sure to set a timer so you don’t forget the fruit there.)
