Preheat your oven to 350°F (175°C). Grease a 9x13 inch baking pan.

Toss the chopped fresh rhubarb with sugar and a little flour. Set aside.

Combine the flour, light brown sugar, and ground cinnamon for the streusel topping. Cut in the cold unsalted butter until it resembles coarse crumbs. Chill.

Whisk together the all-purpose flour, baking soda, and salt in a medium bowl. Set aside.

Cream together the softened unsalted butter and granulated sugar until light and fluffy (about 3-5 minutes).

Add eggs one at a time, mixing well after each. Stir in vanilla extract.

Alternate adding the dry flour mixture and the full-fat sour cream to the wet ingredients. Stir gently until just combined.

Spread half of the cake batter into the prepared pan. Layer with rhubarb filling, dollop on remaining batter, and finish with streusel topping.

Bake for 50-60 minutes or until a skewer inserted in the center comes out clean and top is golden brown.

Allow the cake to cool in the pan for about 20 minutes, then transfer to a wire rack to cool completely before slicing.