Preheat your oven to 350°F (175°C) and prepare a 9-inch square baking pan by greasing it with butter and dusting it with flour.

In a small mixing bowl, combine sour cream and baking soda, stirring until blended. Set aside.

In a large mixing bowl, cream softened butter and sugar together until light and fluffy, about 3-5 minutes.

Add egg and vanilla to the butter-sugar mixture. Beat until integrated, then fold in the sour cream mixture.

Gradually sift in flour and salt, folding gently to combine without overmixing.

Fold in diced rhubarb gently to distribute evenly throughout the batter.

In a separate bowl, combine brown sugar, cinnamon, and nuts (if using) for the topping.

Pour batter into the greased pan, smooth it out, and sprinkle topping evenly over the batter.

Bake for 45-50 minutes, until a toothpick inserted in the center comes out clean.

Allow to cool in the pan for 10-15 minutes before serving.