Put 2 cups flour, ½ cup sugar, and ½ cup cubed butter into a large food processor. Process for 2 minutes until it is blended and crumbly. Reserve 1 cup of this mixture for the topping. Spread the rest into a 9-inch baking dish that has been sprayed with cooking spray and press to form the crust.
Chop the rhubarb into small ½-inch cubes. Put it into a large bowl and add ½ cup flour, ½ cup sugar, and the baking powder. Beat the eggs in a small bowl and mix in the yogurt and vanilla. Pour this over the mixture. Melt the remaining ½ cup butter and add this to the bowl. Stir well to combine and layer it on top of the crust.
Add the brown sugar or coconut sugar, and the cinnamon to the reserved topping. Stir to combine, then layer this on top of the rhubarb mixture. Bake for 45 minutes. Let it cool for 30 minutes. Serve it with whipped cream or ice cream, if you like.