Preheat your oven to 350°F (175°C) and grease a 9x13 inch baking pan with butter or non-stick cooking spray.

In a large mixing bowl, cream together the softened butter and 1 1/4 cups of sugar until the mixture is light and fluffy.

Beat in the egg and the sour cream until the mixture is smooth and well combined.

In a separate bowl, whisk together the flour, baking soda, and salt; gradually add these dry ingredients to the wet mixture until just combined.

Gently fold in the chopped rhubarb using a spatula, ensuring it is evenly distributed throughout the thick batter.

Spread the batter evenly into the prepared baking pan and bake for 40 to 45 minutes, or until a toothpick inserted in the center comes out clean.

While the cake is baking, prepare the sauce by combining the butter, sugar, and heavy cream in a small saucepan over medium heat.

Bring the sauce to a boil and cook for 2 minutes, stirring constantly, then remove from heat and stir in the vanilla extract.

Serve the cake warm, cutting into squares and drizzling each portion generously with the warm butter sauce.