1. Drain the raspberries of the syrup.
2. In a food processor or a blender, puree the berries.
3. Push the puree through a sieve and return the strained puree to the blender or processor.
4. Add the sugar, and blend or process till the sugar dissolves.
5. Add Cointreau if you want.
6. Serve as a cool drink, with ice, if you want.
The drained syrup can be reserved for another use, such as a drink or a gelatin dessert.