Rhubarb Cake with Butter Sauce

Ingredients:

4 cups all-purpose flour
2 cups granulated sugar
4 teaspoons baking powder
½ teaspoon salt
4 cups rhubarb, diced (fresh or frozen)
2 cups milk
6 tablespoons butter, melted
½ cup (1 stick) butter
1 cup sugar
¾ cup heavy cream