For the cake: Position an oven rack in the middle of the oven and preheat to 350 degrees F. Spray a 9-inch loaf pan with nonstick cooking spray.
Sift together the flour and cornstarch. Whisk together the sour cream, oil, sugar and egg in a large bowl until smooth, which will take 1 to 2 minutes.
In a small pitcher or separate bowl, whisk together the buttermilk, vanilla, baking soda and baking powder. Add to the bowl and mix to combine.
Add the dry mixture to the wet ingredients in two batches, stirring until just combined and no lumps of flour remain.
Pour the batter into the prepared loaf pan and bake until a cake tester inserted into the center of the cake comes out clean, 35 to 40 minutes. Put to one side while you make the frosting.
For the gooey chocolate frosting: Combine the milk, chocolate and butter in a saucepot over medium-low heat. Stir until the chocolate has melted and the mixture is smooth, about 3 minutes. Pour the frosting over the still-warm cake and serve immediately, eating with spoons straight from the pan.