Preheat your oven to 350°F (175°C). Grease and flour a 9x5 inch loaf pan, or line with parchment paper leaving an overhang on the long sides for easy removal.

In a large mixing bowl, whisk together the 2 cups all-purpose flour, granulated sugar, baking soda, and salt. Make a well in the center.

In a separate medium bowl, whisk together the eggs, vegetable oil, buttermilk, and vanilla extract until well combined.

Pour the wet ingredients into the well of the dry ingredients. Stir with a rubber spatula until just combined. Be careful not to overmix; a few lumps are fine.

Gently fold in the diced rhubarb until evenly distributed throughout the batter.

In a small bowl, combine 1/4 cup flour, brown sugar, and cinnamon. Cut in the cold butter with a pastry blender or your fingertips until the mixture resembles coarse crumbs. Sprinkle evenly over the top of the batter in the loaf pan.

Pour the batter into the prepared loaf pan. Bake for 55-65 minutes, or until a wooden skewer or toothpick inserted into the center of the loaf comes out clean. If the top is browning too quickly, you can loosely tent it with aluminum foil during the last 15-20 minutes of baking.

Remove the loaf pan from the oven and let it cool in the pan on a wire rack for 10-15 minutes.

Carefully remove the bread from the pan (using the parchment overhang if applicable) and transfer it to the wire rack to cool completely before slicing and serving. This ensures the bread sets properly and is easier to cut.