Preheat the oven to 350°F (175°C) and line an 8×8-inch baking pan with parchment paper, leaving an overhang for easy lifting.
In a medium bowl, using an electric hand mixer, beat 5 tablespoons of the melted coconut oil with the erythritol, whole egg, egg yolks, and mint extract (if using) for about 1 minute until combined and slightly aerated.
Place 4 ounces of the chopped sugar-free dark chocolate and the remaining 2 tablespoons melted coconut oil in a microwave-safe bowl. Microwave at half power in 30-second intervals, stirring between each interval, until the chocolate is smooth and fully melted.
With the mixer running on low, slowly pour the melted chocolate into the egg mixture until thick and well combined.
Add the chocolate protein powder, baking soda, and sea salt to the bowl and stir until incorporated, then vigorously whisk in the almond milk until the batter is smooth.
Finely chop the remaining 1 ounce of chocolate and fold it into the batter.
Spread the batter evenly in the prepared pan and bake for 20–21 minutes, until the edges begin to darken, the center rises slightly, and a toothpick inserted in the center comes out clean.
Let the brownies cool in the pan for 20 minutes, then lift them out using the parchment overhang and transfer to a wire rack to cool completely before slicing.
Slice into 16 squares and serve.
