Prepare the Topping: In a medium mixing bowl, combine brown sugar, flour, melted butter, and salt. Mix until crumbly, about 3-4 minutes.

Mix Dry Ingredients: In a large bowl, whisk flour, baking powder, salt, and sugar until evenly combined, about 2 minutes.

Combine Wet Ingredients: Add sour cream, melted butter, eggs, and vanilla to the dry mixture, stirring until just combined.

Incorporate Rhubarb: Gently fold in the chopped rhubarb until evenly distributed.

Transfer to Pan: Pour the batter into a greased 9-inch baking pan, spreading it evenly.

Top and Bake: Sprinkle crumb topping over the batter and bake in a preheated oven at 350°F (175°C) for about 45 minutes.

Cool and Serve: Let cool for about 10 minutes, then transfer to a wire rack. Serve warm or at room temperature.