Preheat your oven to 350°F (175°C). Line an 8x8-inch baking pan with parchment paper or lightly grease it.
In a large mixing bowl, whisk together the Greek yogurt, granulated sugar, brown sugar, eggs, and vanilla until smooth and well combined.
Sift in the cocoa powder, flour, salt, and baking soda. Stir with a spatula until just combined—do not overmix. Fold in chocolate chips if using.
Pour the batter into the prepared pan and spread evenly. Bake for 22–25 minutes, or until a toothpick inserted comes out with a few moist crumbs. Let cool in the pan before slicing into squares.