Preheat the oven to 350 degrees F. Coat a 9" x 13" baking pan with the oil spray. Cut a 13" x 13" piece of parchment paper to fit the bottom and up 2 of the 4 sides of the pan.
Place the flour, baking powder, spices and salt in a bowl and whisk to combine. Mix in the rhubarb.
In another bowl, beat the butter and brown sugar together until well combined. Mix in the egg, vanilla, buttermilk and sour cream. Mix in the flour/rhubarb mixture, creating a thick batter. Spoon and spread the batter into the pan. Disperse thumbnail-sized pieces of the topping mixture on top of the batter.
Bake 45 minutes, or until a cake tester inserted into the middle of the cake pulls out clean. Cool cake on a baking rack to room temperature. Holding on to the parchment paper, lift cake out of the pan and set on a cutting board. Cut into squares, dust with icing sugar and enjoy.
Note: If using frozen rhubarb, after thawing, remove excess moisture by patting it dry with paper towel.