Preheat the oven to 325 degrees F. Prepare two bread pans, grease, and line with parchment paper or grease and flour the pans
Sift together the flour, baking soda, salt, and cinnamon and set aside.
Wash, peel, and slice the rhubarb.
In a large bowl mix the brown sugar, sugar, and oil until combined.
Add the egg and mix it in. Mix in the vanilla.
Alternate the flour mixture with the buttermilk. Start with adding one-third of the flour mixture, then one-half of the buttermilk, and repeat until all is added.
Divide half of the batter between the two prepared pans. Top the batter with one-half of the topping dividing between the two pans.
Divide the remaining batter between the pans. Then divide the remaining topping between the two pans, sprinkling over top of the batter.
Place the pans in the preheated oven and bake for 55 to 60 minutes or until a wooden pick comes out with just a few moist crumbs.
Remove from the oven and cool on a cooling rack for 5 to 10 minutes then remove the loaves from the pans and cool completely before slicing.