Preheat the oven to 350°F. Grease a 9x13-inch baking dish.
In a medium bowl, combine the chopped rhubarb, 1/2 cup granulated sugar, 1/4 cup flour, salt, and cinnamon. Mix well and set aside.
In another bowl, whisk together the milk, eggs, remaining 1/2 cup granulated sugar, and vanilla extract until smooth.
In a separate bowl, combine 1 cup flour and melted butter to form a crumbly mixture. Press this mixture evenly into the bottom of the prepared baking dish.
Spread the rhubarb mixture over the crust, then pour the milk and egg mixture on top.
Bake for 40-45 minutes, or until the custard is set and the top is lightly golden. Allow to cool completely in the pan before slicing into bars.
Dust with powdered sugar before serving.