Preheat oven to 350° and set rack in middle of oven.
Combine base ingredients in food processor or with pastry blender until fine crumbs. Remove 1 cup of mixture and reserve for top.
Spread mixture in 9 inch square baking pan. Press gently and evenly.
Sift flour, sugar and baking powder. Sprinkle over top of chopped rhubarb and mix well.
Beat eggs. Whisk in yogurt and vanilla. Pour over rhubarb and mix.
Melt butter. Allow to cool slightly. Add to rhubarb mixture and stir to combine. Pour rhubarb over cake base.
Add brown sugar and cinnamon to reserved base mixture. Sprinkle evenly over top of cake.
Bake for 45 - 50 minutes. If making 8 inch cake, bake for about 50 - 60 minutes or until a toothpick inserted in the centre of the cake comes out clean.