Add 2 tablespoons of water and set the heat to medium. As the rhubarb heats, it will start releasing its own juices and breaking down. Stir occasionally to prevent sticking. After about 8 to 10 minutes, the rhubarb will be completely soft and falling apart into a chunky mush.

Remove from heat and let it cool slightly. You can use it as-is for a chunkier texture, or mash it with a fork or potato masher for a smoother consistency. Measure out the cup you need for the recipe and save any extra in the fridge for up to five days. The pulp will have a concentrated tart flavor and a soft, jam-like texture that's perfect for adding moisture to baked goods.

Preheat your oven to 350°F. Line a 9x9-inch square baking pan with parchment paper, leaving some overhang on two sides so you can lift the whole slab out later. You can also just grease the pan well, but parchment makes cleanup stupid easy and prevents any sticking at the corners.

Whisk together the wet ingredients. In a medium bowl, combine granulated sugar, vegetable oil, eggs, and vanilla extract. Whisk hard until the mixture is smooth, glossy, and slightly thickened. This takes about 30 seconds of actual effort. You want the sugar to start dissolving into the oil and eggs so the batter emulsifies properly.

Mix the dry ingredients separately. In another bowl, whisk together all-purpose flour, cocoa powder, baking powder, and sea salt. Whisking breaks up any cocoa clumps and evenly distributes the leavening. Don't skip this step or you'll get pockets of straight cocoa powder or baking powder in the finished bars.

Combine wet and dry. Pour the wet ingredients into the dry ingredients and stir with a spatula just until you don't see any more flour streaks. The batter will be thick and fudgy. Stop mixing as soon as it comes together. Overmixing develops gluten and makes the bars tough and cakey instead of dense and gooey.

Fold in the rhubarb and mix-ins. Add 1 cup rhubarb pulp, 1/2 cup finely chopped fresh rhubarb, and chocolate chips. Fold everything together gently with your spatula.

Scrape the batter into your prepared pan and use the spatula to spread it into an even layer. Press the batter into the corners and smooth the top as much as you can. It doesn't have to be perfect, but an even layer bakes more consistently.

Bake for 25 to 30 minutes. Start checking at 25 minutes. The edges should be set and pulling away slightly from the pan. The center should still look a little underdone and jiggly. A toothpick inserted near the center should come out with moist crumbs clinging to it, not wet batter, but definitely not clean. This is a gooey bar, not a cake. Underbaking slightly is better than overbaking.

Cool completely in the pan. This is non-negotiable. Let the bars cool to room temperature before you even think about slicing. As they cool, the structure sets and the gooey center firms up just enough to slice cleanly. If you cut them warm, they'll be a melty, delicious mess. Which honestly isn't the worst problem, but you won't get clean squares.

Once cooled, lift the whole slab out using the parchment overhang, set it on a cutting board, and slice into 16 squares with a sharp knife. Wipe the knife clean between cuts for the cleanest edges.