Preheat your oven to 350°F (175°C). Grease and flour a baking pan.
In a mixing bowl, cream together the softened butter and granulated sugar until light and fluffy. Add eggs one at a time, mixing well after each addition. Stir in the vanilla extract and sour cream.
In another bowl, whisk together the flour, baking powder, baking soda, and salt. Gradually add the dry ingredients to the wet mixture, mixing until just combined. Fold in the chopped rhubarb.
Pour the batter into the prepared baking pan.
For the crumb topping, mix together the flour, oats, brown sugar, and cinnamon in a bowl. Cut in the cold butter until the mixture resembles coarse crumbs. Sprinkle evenly over the cake batter.
Bake in the preheated oven for 40 minutes or until a toothpick inserted into the center comes out clean. Allow to cool before serving.
Notes
You can serve this cake warm or at room temperature. It pairs well with coffee or tea.