Preheat your oven to 350°F (175°C). Grease and flour a baking pan.
In a mixing bowl, combine the flour, baking powder, baking soda, and salt.
In another bowl, cream the softened butter and granulated sugar until light and fluffy. Add the eggs one at a time, then stir in the vanilla extract and sour cream.
Gradually mix the dry ingredients into the wet ingredients. 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 crumbly. Sprinkle over the cake batter.
Bake for 40 minutes or until a toothpick inserted in the center comes out clean. Let cool before serving.
Notes
Enjoy this cake with a cup of coffee or tea. You can substitute other fruits for rhubarb if desired.