Heat oven to 325°F and move oven rack to the middle position. Cut out a piece of parchment paper to fit the bottom of a 9x9" baking pan (trace around the bottom of the pan on a piece of parchment with pencil, and cut it out). Generously oil the sides of the pan, and lay the parchment on the bottom of the pan (gently oil the bottom of the pan to help the parchment stick). Set the prepared pan aside.
Sift together the 1 cup all purpose flour, 1 cup granulated sugar, 2/3 cup cocoa powder, 1/2 cup powdered sugar, and 1/2 teaspoon kosher salt, in a medium bowl. Whisk gently until mixed.
In a large bowl, whisk together the 3 large eggs, 1/2 cup canola oil, 2 Tablespoons water,1 teaspoon vanilla extract, and 1/2 teaspoon instant coffee powder. Pour the dry ingredient mixture over the egg mixture and fold together until mixed. Fold in 1/2 cup dark (semisweet) chocolate chips (if using) until incorporated.
Spread the batter (it will be thick) into the prepared baking pan. Do this carefully so that the parchment doesn't slide around. Bake for 30-35 minutes, until the top appears dry, but the center will not be set (a toothpick will still have a chocolatey coating). Do not overbake.
Cool completely on a wire rack before cutting. Store leftovers in an airtight container at room temperature for up to 2 days, or in the freezer.