Preheat your oven to 350°F (175°C) and position the rack in the middle. Grease and flour a 9x13 inch baking pan, or line with parchment paper with overhanging sides for easy removal.
Wash the rhubarb stalks thoroughly and trim off the leaves (these contain oxalic acid and are toxic). Cut the stalks into ½-inch pieces. If your rhubarb is particularly stringy, you can pull off the longer fibers as you would with celery.
In a large bowl, whisk together the flour, sugar, baking soda, and salt until well combined.
In a separate bowl, whisk together the buttermilk, oil, eggs, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients, stirring just until combined. Be careful not to overmix – stop when no dry flour pockets remain. Fold in the chopped rhubarb gently.
Mix all topping ingredients in a bowl until combined and crumbly. The mixture should clump when pressed together but still be easy to sprinkle.
Pour the batter into the prepared pan, spreading it evenly. Sprinkle the crumb topping uniformly over the batter.
Bake for 45-50 minutes, or until a toothpick inserted in the center comes out clean with just a few moist crumbs.
Allow the cake to cool in the pan for at least 20 minutes before serving. This cooling period allows the structure to set and flavors to meld.