Preheat your oven to 325°F and grease two loaf pans (approximately 8x4 inches each) with butter or cooking spray, paying special attention to the corners and bottom seams. While the oven heats, chop your rhubarb into 1/2-inch pieces and your pecans into 1/4-inch pieces, keeping them in separate bowls. This prep work ensures you're ready to move quickly through the mixing stages without interruption.

In a large mixing bowl, whisk together the brown sugar, vegetable oil, and room temperature egg until the mixture is smooth and well combined—this should take about 1-2 minutes of whisking. In a separate bowl, combine the buttermilk, baking soda, salt, and vanilla, stirring gently until the baking soda is fully dissolved and the mixture is uniform. The room temperature egg will blend more smoothly with the oil, creating a better emulsion than a cold egg would.

Pour the buttermilk mixture from Step 2 into the sugar mixture from Step 2, stirring gently to combine. Add the flour and mix with a spatula or wooden spoon until just combined—the batter should be slightly lumpy at this point. Overmixing at this stage will develop gluten and make the bread tough, so stop as soon you don't see dry flour streaks.

Gently fold the chopped rhubarb and pecans into the batter from Step 3 using a spatula, stirring just until they're evenly distributed throughout. I like to reserve a few small pecan pieces to sprinkle on top later—they add a nice visual appeal and extra crunch to the finished loaves. Be gentle during this folding to avoid crushing the rhubarb pieces, which will help them stay distinct in texture.

Divide the batter evenly between the two prepared pans from Step 1, smoothing the tops gently with a spatula. In a small bowl, stir together the sugar, melted butter that has cooled to room temperature, and cinnamon until you have a crumbly mixture. Sprinkle this topping evenly over both loaves, dividing it equally between them.

Place both pans in the preheated 325°F oven and bake for 45 minutes, until a toothpick inserted in the center comes out with just a few moist crumbs (not wet batter). The loaves should be golden brown on top. Remove from the oven and let the loaves cool in their pans for 10 minutes before running a thin knife around the edges and turning them out onto a wire rack to cool completely.