In a small bowl, combine flour, brown sugar, cinnamon, and a pinch of salt. Cut in the butter until the mixture resembles coarse crumbs. Refrigerate if necessary.

Preheat your oven to 350°F (175°C) and grease a 9-inch square baking pan with butter or line it with parchment paper.

In a large mixing bowl, whisk together flour, baking powder, baking soda, and salt. In another bowl, cream the butter and brown sugar until light and fluffy, then add eggs one at a time, followed by sour cream and vanilla until smooth.

Spread half of the batter into the prepared baking pan. Toss rhubarb with sugar and a little flour, sprinkle it over the batter, then dollop the remaining batter on top.

Sprinkle the chilled streusel over the top of the batter evenly.

Bake for 40-50 minutes or until a toothpick inserted comes out with moist crumbs. Watch for doneness, especially with dark pans.

Let the cake cool in the pan for at least 20 minutes before slicing.