Instructions

1. 1

   Preheat the oven to 350°F and grease a 9-inch square baking pan. Line the bottom with parchment if you want especially clean slices.

2. 2

   In a medium bowl, whisk together the flour, baking powder, baking soda, and salt until evenly combined.

3. 3

   In a separate bowl, mix the crumb topping ingredients with a fork or pastry cutter until the mixture forms chunky crumbs with some pea-sized butter pieces still visible. Refrigerate while you prepare the batter so the topping stays crisp during baking.

4. 4

   Beat the butter and sugar until the mixture looks pale and fluffy, about 2 to 3 minutes. Add the eggs one at a time, then stir in the vanilla extract.

5. 5

   Mix in half of the dry ingredients, followed by the sour cream, then the remaining dry ingredients. The batter should look thick and smooth without dry streaks.

6. 6

   Fold in the chopped rhubarb gently so the pieces stay evenly distributed throughout the batter.

7. 7

   Spread the batter into the prepared pan and smooth the top with a spatula. Sprinkle the crumb topping evenly over the surface, lightly pressing larger clumps into place.

8. 8

   Bake for 40 to 50 minutes, or until the top is golden brown and the center springs back lightly when touched. A toothpick inserted near the middle should come out with a few moist crumbs but no wet batter.

9. 9

   Let the cake cool for at least 20 minutes before slicing. The crumb topping firms up as it cools, making cleaner slices easier.