A moist and tender coffee cake filled with tart rhubarb and topped with a buttery cinnamon streusel, perfect for breakfast or dessert.

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Preheat oven to 350°F (175°C). Grease a 9×13-inch baking dish.
In a small bowl, combine chopped rhubarb with 1/2 cup granulated sugar. Set aside.
In a large bowl, cream together butter and 1 cup brown sugar until light and fluffy. Beat in the egg and vanilla.
In another bowl, whisk together 2 cups flour, baking soda, and salt. Gradually add to the creamed mixture, alternating with sour cream.
Fold in the rhubarb mixture and walnuts if using. Spread batter evenly into the prepared baking dish.
For the streusel topping, combine 1/3 cup flour, 1/3 cup brown sugar, and cinnamon. Cut in the cold butter until crumbly. Sprinkle evenly over the batter.
Bake for 40–45 minutes or until a toothpick inserted in the center comes out clean.
Cool in the pan before serving.