In a saucepan, combine chopped rhubarb, water, sugar, and lemon juice. Cook over medium heat until rhubarb is soft, about 10 minutes.

Strain the mixture through a fine sieve to remove solids, reserving the liquid.

In a separate saucepan, mix cornstarch, water, sugar, and cream of tartar. Cook over low heat, stirring constantly until thickened.

Once thick, add the reserved rhubarb liquid and rosewater. Continue to cook, stirring, until the mixture reaches 240°F (115°C) on a candy thermometer.

Pour the mixture into a greased baking dish and let it set at room temperature for at least 4 hours or until firm.

Once set, cut into squares and toss in a mixture of powdered sugar and cornstarch to coat.