Preheat your oven to 350°F (175°C). Lightly grease an 8×8-inch baking pan or line it with parchment paper, leaving some overhang on two sides to make it easy to lift the bars out for slicing.

In a medium mixing bowl, whisk together the all-purpose flour, granulated sugar, cocoa powder, baking soda, and salt until evenly combined and no streaks of cocoa remain.

Use the back of a spoon to make three small wells in the dry mixture. Pour the vanilla extract into one well, the vinegar into the second, and the neutral oil into the third.

Pour the water or cooled coffee over the dry ingredients and wells. Whisk gently at first to avoid cocoa puffing out, then whisk until the batter is mostly smooth. A few small lumps are fine. The batter will be looser than brownie batter but thicker than crepe batter.

If using, gently fold in the chocolate chips or chunks and chopped nuts with a spatula until evenly distributed.

Pour the batter into the prepared 8×8-inch pan. Use a spatula to scrape the bowl and smooth the top. Tap the pan once or twice on the counter to release any large air bubbles.

Bake at 350°F (175°C) for 25–30 minutes, or until the top looks set and slightly matte and a toothpick inserted in the center comes out with a few moist crumbs but no wet batter. Start checking around 23–24 minutes, as ovens vary.

Place the pan on a cooling rack and let the cake cool completely in the pan, at least 45–60 minutes. This helps the bars set so they slice cleanly. For faster cooling after it reaches room temperature, you can chill the pan briefly in the refrigerator.

In a small bowl, whisk together the powdered sugar, cocoa powder, and a pinch of salt. Add the vanilla extract and 2 tablespoons of water, strong coffee, or plant milk. Whisk until smooth, adding more liquid 1 teaspoon at a time as needed until the glaze is pourable but not runny.

Pour the glaze over the fully cooled cake and spread it gently with an offset spatula or the back of a spoon. Let the glaze set for 20–30 minutes (it may remain slightly soft and fudgy). Use the parchment overhang to lift the cake from the pan onto a cutting board. Slice into 9 large bars or 12–16 smaller squares, wiping the knife with a warm, damp cloth between cuts for neat edges.

Serve the bars at room temperature. Store in an airtight container at room temperature for 2–3 days or in the refrigerator for up to 5 days. For longer storage, freeze the sliced bars on a sheet pan until firm, then transfer to an airtight container and freeze for up to 2 months.