Preheat the Oven: Preheat the oven to 350°F (175°C) so it is ready when the brownie batter is prepared.
Prepare the Baking Pan: Line an 8 × 8-inch square baking pan with parchment paper, ensuring the base is covered. Set the prepared pan aside.
Blend the Avocado Mixture: Place the ripe avocados, eggs, and unsweetened cocoa powder in a mixing bowl or food processor. Blend until the mixture is smooth and creamy, creating the fudgy base for the brownies. Incorporate the Dry Ingredients: Add the almond flour, arrowroot starch or coconut flour, and xylitol or chosen sugar substitute. Blend or mix until the batter is smooth. Use a spatula to scrape down the sides of the bowl and mix again if needed. Avoid overmixing to help preserve the brownies' fudgy texture. Transfer the Batter: Transfer the blended mixture to a mixing bowl if you used a food processor, making it easier to incorporate the remaining ingredients.
Add the Flavorings and Leavening: Stir in the vanilla extract, baking soda, sea salt, and a pinch of ground cinnamon until evenly distributed throughout the batter.
Fill the Baking Pan: Pour the brownie batter into the prepared pan. Use a spatula to spread it evenly into the corners and smooth the surface.
Melt the Dark Chocolate: Place the dark chocolate chips in a microwave-safe bowl and melt them in the microwave, or melt them on the stovetop. Stir as needed until the chocolate is melted and smooth.
Prepare the Tahini Topping: Add the tahini and optional maple syrup to the melted chocolate. Stir gently until the ingredients are combined into a smooth, evenly blended mixture.
Add the Topping: Spread the chocolate-tahini mixture over the brownie batter, covering the surface. If desired, sprinkle chopped nuts over the top.
Bake the Brownies: Place the pan in the preheated oven and bake for 25–30 minutes. Check the center with a toothpick; the brownies are ready when it comes out clean.
Cool and Set: Remove the pan from the oven and let the brownies cool on the counter for 10 minutes. Transfer them to the refrigerator for another 5 minutes to help them set before slicing.
Slice and Store: Cut the cooled brownies into portions and serve. For best storage, keep the remaining brownies in the refrigerator or freeze them.