Combine rhubarb and 1 1/2 teaspoons sugar in a large bowl; toss to combine. Allow to sit for at least 30 minutes or until rhubarb begins to release its juices.

Drain accumulated juice from rhubarb; discard juice. Add cornstarch to rhubarb and toss until thoroughly coated. Set aside.

Preheat the oven to 350 degrees F (175 degrees C). Line 2 baking sheets with parchment.

Beat butter and 3/4 cup sugar together in a large bowl until fluffy, about 5 minutes. Add in egg and vanilla and beat for another 3 minutes. Add in flour, cream of tartar, baking soda, salt, and nutmeg and mix until just combined. Fold in rhubarb.

In a small bowl, mix together 3 tablespoons sugar and cinnamon.

Scoop out 1 1/2 tablespoons dough, roll into a ball, and roll ball in cinnamon sugar until thoroughly coated. Place dough balls 1 1/2 inches apart on the prepared baking sheets. Repeat with remaining dough.