Preheat your oven to 350°F. Grease a 9-inch pie dish with butter.
In a large mixing bowl, combine the chopped rhubarb, sugar, flour, and salt. Stir until the rhubarb is well coated.
In another bowl, whisk together the eggs, milk, vanilla extract, and cinnamon until fully combined.
Gradually pour this mixture over the rhubarb mixture, stirring gently to combine.
Pour the entire mixture into the prepared pie dish. Drizzle the melted butter over the top.
Bake in the preheated oven for 40-45 minutes, or until the custard is set and the top is lightly golden.
Allow to cool for at least 15 minutes before serving.
