In a mixing bowl, combine brown sugar, all-purpose flour, melted butter, and a pinch of salt. Mix until the mixture resembles coarse crumbs. Set aside.

In a separate bowl, whisk together all-purpose flour, salt, baking powder, and sugar until well combined. Set aside.

In another bowl, mix together sour cream, melted butter, eggs, and vanilla extract until smooth. Set aside.

Gently fold the wet mixture into the dry ingredients until just combined. Fold in chopped rhubarb.

Transfer the batter into a well-buttered and parchment-lined 9-inch round baking pan. Sprinkle the prepared crumb topping evenly over the batter.

Preheat the oven to 350°F (175°C). Place the baking pan in the oven and bake for approximately 45 minutes.

Once baked, remove from the oven and let cool in the pan for about 10 minutes before transferring to a wire rack to cool completely.

Serve warm, optionally with whipped cream or vanilla ice cream.