In a large bowl or the bowl of a standing mixer fitted with the paddle attachment, stir together the flour, sugar, baking powder, cinnamon, baking soda, and salt. Cut the butter in with a fork or pastry blender or place the mixer on low speed. Continue working in the butter until some pieces are the size of oat flakes and some are the size of your thumbnail. The butter pieces may look large, but this is OK. By hand with a spatula, gently fold in the rhubarb.
Place rack in the center of oven and preheat oven to 375 degrees F. Line a large baking sheeting with parchment paper and set aside. Dice butter into small pieces and place in the freezer while you prepare the other ingredients.