Preheat oven to 350°F (180°C) and line an 8x8-inch pan with parchment paper.
Melt the chopped chocolate in the microwave in short bursts, stirring until smooth. Let cool slightly.
Whisk Greek yogurt and sweetener into the melted chocolate until smooth.
Add flour, cocoa powder, and baking soda. Mix gently until just combined.
Pour in the milk and stir until the batter is thick and fully mixed.
Fold in chocolate chips, if using.
Transfer the batter into the prepared pan and smooth the top with a spatula.
Bake for 25–30 minutes or until a toothpick comes out with moist crumbs.
Let cool in the pan for 10 minutes, then transfer to a rack to cool completely.
Slice into squares and enjoy!