Preheat your oven to 350°F (175°C). Grease a 9×13-inch baking pan lightly with butter or cooking spray.

Rinse your fresh rhubarb under cool water and chop it into bite-sized pieces. If using frozen rhubarb, chop while still frozen.

In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt until fully combined.

Add the softened unsalted butter, eggs, vanilla extract, and buttermilk to the dry mixture. Mix gently until just combined.

Fold the chopped rhubarb into the batter using a spatula, ensuring it is evenly distributed throughout the mixture.

Pour the batter into the prepared baking pan, spreading it evenly across the bottom.

In a small bowl, mix together the brown sugar, ground cinnamon, and chopped walnuts. Sprinkle evenly over the top of the batter.

Bake the cake in the preheated oven for 35-40 minutes, until a toothpick inserted in the center comes out clean or with a few moist crumbs.

Allow the cake to cool in the pan for about 10 minutes before serving.