Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 9-inch round cake pan or a 9x13 inch baking dish. For a round pan, lining the bottom with parchment paper is recommended.
Step 2: In a large bowl, whisk together the all-purpose flour, granulated sugar, baking soda, baking powder, and salt until well combined and no lumps remain.
Step 3: In a separate medium bowl, whisk together the melted unsalted butter, buttermilk, eggs, and vanilla extract until smooth.
Step 4: Pour the wet ingredients into the dry ingredients. Mix with a spatula or wooden spoon until just combined. Be careful not to overmix; a few streaks of flour are fine.
Step 5: Gently fold in the chopped fresh rhubarb, distributing it as evenly as possible throughout the batter.
Step 6: Pour the batter into your prepared cake pan, spreading it evenly with the spatula.
Step 7: Bake for 45-55 minutes, or until a wooden skewer or toothpick inserted into the center of the cake comes out clean. The top should be golden brown and spring back when lightly touched.
Step 8: Remove the cake from the oven and let it cool in the pan on a wire rack for 10-15 minutes before inverting it onto the rack to cool completely. This helps prevent the cake from breaking.