Preheat the oven to 350F. Line an 8 x 8-inch square baking pan with parchment paper. (Double-check your pan size—a 9 x 9-inch pan won't work for this recipe.)
In a large mixing bowl, beat together the Stevia In The RawⓇ, coconut sugar, tahini, and vanilla for 1 minute using a stand mixer with the paddle attachment (or a hand mixer with the beater attachments if you don't have a stand mixer).
In a medium mixing bowl, whisk together the oat flour, cocoa powder, ground flaxseed, baking powder, and sea salt. Place the water within reach of the mixer.
With the mixer on low, slowly add the oat flour mixture to the tahini mixture. Then, immediately add the water, increase to medium speed, and beat for 15 seconds, or until incorporated. The dough should be moist and pulled together in large mounds as it moves around the mixing bowl, not dry and broken into small crumbles. Note: If the dough appears dry and crumbly (it likely will if you're using a hand mixer), add another 1–2 tablespoons water. A little liquid goes a long way in this recipe, so start with 1 tablespoon and mix again before determining whether or not a second tablespoon is needed. Adding more liquid may increase the baking time by a minute or two, so adjust as needed.
Fold the chocolate chunks into the batter/dough.
Scoop the brownie dough into the lined pan, and use your fingers or a spatula to press and spread it into an even layer.
Bake for 14–18 minutes, or until the edges appear slightly crackled and a toothpick inserted into the center comes out mostly clean with a few moist crumbs at the tip. Lean toward underbaking if you're questioning doneness, as this will ensure a dense, fudgy texture.
Let cool completely in the pan. In a pinch, cool at room temperature for 5 minutes and then transfer to the refrigerator to expedite the cooling process.