Preheat the oven to 350°F. In a large bowl, whisk together the flour, sugar, baking powder, and salt. Add the diced rhubarb, milk, and melted butter. Stir just until combined.
Pour the batter into a greased 9 x 13 inch baking pan. Bake for 45 minutes or until a toothpick inserted comes out clean.
Remove from the oven and cool.
To make the butter sauce, combine the butter, sugar, and cream in a heavy saucepan. Bring to a boil over medium heat and boil for 1 minute, stirring frequently. Remove from heat and let cool slightly.
To serve, cut pieces of cake and serve with a generous amount of warm butter sauce. The sauce can be gently reheated on the stove or in the microwave if necessary.