Grease the sides and bottom of a 9-inch springform pan well with butter or cooking spray. Cut out a parchment paper circle to fit the bottom of the pan and place it inside.
In a small bowl, mix 1/2 cup of sugar with chopped rhubarb and let sit to activate juices.
Wrap the bottom of the springform pan in tin foil and place it on a baking sheet. Then, melt 1/4 cup butter and pour it into the prepared springform pan.
Arrange the chopped rhubarb in a flat layer on the bottom of the buttered springform pan. Let it sit while you prepare the batter.
In a bowl, sift together the flour, baking powder, and salt. Set aside.
In another bowl, cream together 1 cup of softened butter and 1 1/2 cups of granulated sugar until light and fluffy. Beat in the eggs, one at a time, then stir in the vanilla extract.
Gradually mix in the flour mixture, alternating with the milk, until just combined. Be careful not to overmix.
Pour the batter over the rhubarb in the springform pan, spreading it evenly.
Place the springform pan in the preheated oven and bake for 50-60 minutes, or until a toothpick inserted into the center comes out clean.
Allow the cake to cool in the pan for 10 minutes, then carefully release the sides of the springform pan and invert the cake onto a serving plate. If desired, you can top it with your favorite red jam or jelly to enhance the color and sweetness. Just heat the jam slightly in the microwave and brush on.
Serve as is or with a dollop of whipped topping and more jam!