Preheat your oven to 350°F (175°C). This ensures that your brownies bake evenly from the start.
Line a 9x9-inch baking pan with parchment paper, leaving some overhang for easy removal later. Lightly grease the paper to prevent sticking.
In a microwave-safe bowl, melt 4 ounces of dark chocolate and 2 tablespoons of coconut oil together. Heat in 30-second intervals, stirring in between until smooth and fully melted. Let it cool slightly.
In a large mixing bowl, combine 2/3 cup of nonfat vanilla Greek yogurt, 1/4 cup of coconut sugar, and 2 large eggs. Whisk until smooth and well combined.
Pour the melted chocolate mixture into the wet ingredients. Stir until everything is combined.
In a separate bowl, whisk together 1/2 cup of cocoa powder, 1 tablespoon of cornstarch, 1/2 teaspoon of baking soda, and 1/4 teaspoon of salt.
Gradually add the dry ingredients to the wet mixture, using a spatula to fold them together. Be careful not to overmix; just stir until the dry ingredients are fully incorporated.
Chop the remaining 1 ounce of dark chocolate into small pieces and fold them into the brownie batter for added texture and richness.
Pour the batter into the prepared baking pan and spread it evenly. Bake in the preheated oven for 20-25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs.
Once baked, remove the brownies from the oven and let them cool in the pan for about 10 minutes. Use the parchment overhang to lift them out and let them cool completely on a wire rack before cutting into squares.
