Step 1: Preheat your oven to 375°F (190°C). Line an 8x8 inch (20x20 cm) baking pan with parchment paper, leaving an overhang on two sides to easily lift the brownies later. Lightly butter or grease the parchment.
Step 2: In a medium-sized bowl, whisk together the sugar, vegetable oil, eggs, and vanilla extract until the mixture is smooth and well combined.
Step 3: In a separate large bowl, whisk together the all-purpose flour, cocoa powder, baking powder, and sea salt, ensuring there are no lumps.
Step 4: Pour the wet ingredients into the dry ingredients. Stir gently with a spoon or spatula until just incorporated. The batter will be thick.
Step 5: Gently fold in the chopped rhubarb, chocolate chips, and chopped walnuts (if using) into the brownie batter. Be careful not to overmix.
Step 6: Transfer the batter to the prepared baking pan, spreading it evenly with your spatula. Ensure the rhubarb and chocolate chips are distributed throughout.
Step 7: Bake for 20-25 minutes, or until a toothpick inserted into the center comes out with moist crumbs attached (not wet batter).
Step 8: Allow the brownies to cool completely in the pan for at least 30 minutes before lifting them out using the parchment paper overhang and cutting them into squares. This helps them set and become perfectly fudgy.