Preheat the oven to 350?F (175?C).

In a saucepan, melt the butter over medium heat. Stir in the brown sugar until dissolved.

Add the chopped rhubarb and cook for 5 minutes until slightly softened. Pour the mixture into the bottom of the greased cake pan.

In a mixing bowl, whisk together the flour, baking powder, baking soda, and salt.

In another bowl, combine the granulated sugar, buttermilk, vegetable oil, egg, and vanilla extract. Mix until smooth.

Gradually add the dry ingredients to the wet ingredients, stirring until just combined.

Pour the cake batter over the rhubarb topping in the pan.

Bake in the preheated oven for 30-35 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.