Preheat the oven to 350°F (175°C). Grease a 9x13-inch baking dish.
In a large bowl, combine the flour, 1 cup sugar, salt, baking powder, and baking soda. Mix well.
Add the softened butter, egg, and buttermilk to the dry ingredients. Mix until just combined.
Spread the batter evenly in the prepared baking dish.
In another bowl, combine the chopped rhubarb, sour cream, 1/2 cup sugar, 2 eggs, and vanilla extract. Mix until smooth.
Pour the rhubarb custard mixture over the batter in the baking dish.
Bake for 40-45 minutes, or until the top is set and lightly golden.
Allow to cool for at least 15 minutes before slicing and serving.