Preheat oven to 350°F (175°C). Grease and flour a 9-inch springform pan.

In a medium bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt for the cake batter.

In a separate large bowl, cream together the softened butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla extract and sour cream until just combined.

Gradually add the dry ingredients to the wet ingredients, mixing on low speed until just incorporated. Do not overmix. Gently fold in the chopped rhubarb pieces.

To make the streusel, combine the flour, brown sugar, and cinnamon in a small bowl. Cut in the cold butter using your fingertips or a pastry blender until coarse crumbs form. Sprinkle the streusel evenly over the batter in the prepared pan.

Bake for 50 to 55 minutes, or until a wooden skewer inserted into the center comes out clean. Let the cake cool in the pan for 15 minutes before releasing the sides of the springform pan. Cool completely on a wire rack before serving.