Warm up the oven: Go ahead and preheat your oven to 175°C (350°F). I always like to do this first so it’s ready when the batter is. Lightly grease a standard 9×5-inch loaf pan or line it with parchment for easy removal.
Mix the wet ingredients: In a large mixing bowl, combine the brown sugar and oil until smooth, I use a whisk or wooden spoon. Mix in the egg, vanilla, and buttermilk, and stir until the mixture becomes smooth and evenly combined.
Prepare the dry mix: In another bowl, whisk together the flour, baking soda, salt, and cinnamon. This helps evenly distribute the baking soda and spice so you don’t end up with clumps.
Bring it all together: Slowly stir the dry ingredients into your wet mixture. Don’t overmix, it’s totally fine if the batter looks a little lumpy. I’ve learned that a gentle hand makes softer bread.
Fold in the good stuff: Gently fold in your chopped rhubarb and nuts (if using). I like to do this with a spatula so the rhubarb stays intact and spread evenly throughout.
Bake and cool: Carefully transfer the batter into your greased loaf pan, then gently level the surface. Let it bake for 40 to 50 minutes, or until a skewer inserted into the middle comes out without any wet batter. Let it rest in the pan for 5-10 minutes, then transfer it to a wire rack to cool completely before slicing.