Preheat your oven to 375°F (190°C) and prepare an 8x8 inch baking pan with parchment paper, lightly buttering it.

In a medium mixing bowl, combine granulated sugar, vegetable oil, large eggs, and vanilla extract. Whisk until smooth and creamy for about 2 minutes.

In a large bowl, whisk together all-purpose flour, cocoa powder, baking powder, and sea salt until combined.

Gently fold the wet ingredients into the dry mixture, then add the diced rhubarb, dark chocolate chips, and walnuts until evenly distributed.

Pour the batter into the prepared pan and bake for 20-25 minutes. A toothpick inserted should come out with a few moist crumbs.

Let the brownies cool in the pan for about 10 minutes before lifting them out using the parchment overhang. Cool completely on a wire rack before slicing.