Preheat your oven to 350°F (175°C). Line an 8x8-inch pan with parchment, leaving overhang for easy lifting.
Lightly mist with cooking spray.
In a medium bowl, whisk together 3/4 cup plain Greek yogurt, 2 large eggs, 1 teaspoon vanilla, and 2 tablespoons neutral oil until smooth.
In a separate bowl, combine 1/2 cup unsweetened cocoa powder, 3/4 cup granulated sweetener (or to taste), 3/4 cup all-purpose flour, 1 teaspoon baking powder, 1/4–1/2 teaspoon salt, and 1/2 teaspoon espresso powder (optional).
Fold the dry ingredients into the wet using a spatula. Mix just until no flour streaks remain. The batter will be thick.
Stir in 2–3 tablespoons dark chocolate chips if using.
This keeps calories reasonable while adding gooey pockets.
Spread the batter evenly in the prepared pan. Smooth the top with a spatula.
Bake for 20–25 minutes, until the center is set but still soft and a toothpick comes out with a few moist crumbs.
Cool in the pan for at least 20–30 minutes. For clean slices and the fudgiest texture, chill for 1 hour before cutting.