Preheat your oven to 350°F (175°C). Take a 9-inch round cake pan (or similar size, like an 8x8 square) and pour the 1/2 cup of melted butter evenly over the bottom.

Sprinkle the 1 cup of packed brown sugar evenly over the melted butter. Arrange the 3 cups of chopped rhubarb pieces neatly and closely together over the sugar and butter mixture in the pan.

Prepare the batter: In a medium bowl, whisk together the flour, baking powder, and salt. Set aside.

In a separate large bowl, cream together the 1/2 cup softened butter and granulated sugar until light and fluffy. Beat in the eggs one at a time, followed by the vanilla extract.

Alternate adding the dry ingredients mixture and the milk to the wet ingredients, beginning and ending with the dry ingredients. Mix only until just combined; do not overmix.

Carefully spoon the cake batter evenly over the layer of rhubarb in the prepared pan, trying not to disturb the fruit underneath.

Bake for 40 to 45 minutes, or until a wooden skewer inserted into the center of the cake comes out clean. Allow the cake to cool in the pan on a wire rack for exactly 10 minutes.

After 10 minutes, place a serving platter upside down over the cake pan. Invert the pan and platter together quickly. If any rhubarb sticks, gently place it back onto the cake surface. Let it cool slightly before slicing and serving.