Preheat the oven to 350°F (175°C). In a mixing bowl, combine the chopped rhubarb, brown sugar, and melted butter. Mix well and spread evenly in the bottom of the prepared cake pan.

In another mixing bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt.

In a separate bowl, combine the milk, vegetable oil, egg, and vanilla extract. Mix well.

Pour the wet ingredients into the dry ingredients and stir until just combined.

Pour the cake batter over the rhubarb mixture in the cake pan, spreading it evenly.

Bake in the preheated oven for 35-40 minutes, or until a toothpick inserted into the center comes out clean.

Let the cake cool for 10 minutes, then invert onto a serving plate.