Preheat oven to 350°F (175°C). Grease and flour a 9x13 inch baking pan.

Prepare the rhubarb filling: In a medium bowl, toss chopped rhubarb with granulated sugar, cornstarch, and vanilla extract. Set aside.

Prepare the cake batter: In a large bowl, cream together the softened butter and granulated sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla extract.

In a separate bowl, whisk together the flour, baking soda, and salt. In another small bowl, stir together the sour cream.

Add the dry ingredients to the wet ingredients alternately with the sour cream, beginning and ending with the dry ingredients. Mix until just combined; do not overmix.

Spread the cake batter evenly into the prepared baking pan. Spoon the rhubarb filling evenly over the batter.

Bake for 45-50 minutes, or until a wooden skewer inserted into the center comes out clean. Let cool slightly.

Prepare the butter sauce: While the cake is still warm, melt the butter in a small saucepan over medium heat. Stir in the brown sugar and heavy cream. Bring to a simmer and cook for 2 minutes, stirring constantly. Remove from heat and stir in the vanilla extract.

Pour the warm butter sauce over the warm cake. Serve warm.