Put all the ingredients except butter and vanilla extract in a bowl and mix to distribute evenly. Now stir in the melted butter and vanilla until all the dry bits are moistened. Press the crumble to the bottom and set aside.

Cut the rhubarb into small pieces and mix it with all of the other ingredients.

Preheat the oven to 180°C. Grease a 20 x 20 cm square baking pan or line it with parchment paper.

In a bowl beat butter and sugar together until light and fluffy, about 3 min on high speed. Add the egg and mix until well incorporated, then add in the sour cream and vanilla and mix again, for half a minute. In another bowl mix dry ingredients – flour, baking soda, baking powder and salt. Add them to the butter mixture and mix just until everything is incorporated. Spread the batter on the bottom of a prepared baking pan. Evenly distribute the rhubarb mixture on top. Then sprinkle the crumble you made over the rhubarb, you should have some big and some smaller crumbs. Bake for 40-50 min, check with an inserted toothpick or knife - if it comes out with just a few moist crumbs it’s done.

Leave to cool for a bit, cut and enjoy :)!