Preheat your oven to 350°F (175°C) and position the rack in the center. Grease and flour a 9×13-inch baking pan, or line with parchment paper, leaving an overhang on two sides.
Wash the rhubarb stalks and trim off the leaves (which contain toxic compounds). Cut the stalks into ½-inch pieces. Toss the rhubarb with 1 tablespoon of the measured flour to prevent the pieces from sinking during baking.
In a large bowl, whisk together the flour, 1 cup of sugar, baking soda, and salt until well combined.
In a separate medium bowl, whisk together the buttermilk, vegetable oil, eggs, and vanilla until smooth and well incorporated.
Make a well in the center of the dry ingredients and pour in the wet ingredients. Stir just until combined, being careful not to overmix. Gently fold in the chopped rhubarb, distributing it evenly throughout the batter.
In a small bowl, mix the remaining sugar with the ground cinnamon. Pour the batter into your prepared pan, spreading it evenly. Drizzle the melted butter over the top, then sprinkle with the cinnamon-sugar mixture.
Bake for 45-50 minutes, or until a toothpick inserted into the center comes out clean with a few moist crumbs. The internal temperature should reach about 205°F for the ideal moisture content.
Allow the cake to cool in the pan for 15 minutes before transferring to a wire rack to cool completely.