Preheat the oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper, leaving an overhang to lift the brownies out later.
In a medium bowl, using an electric hand mixer, beat together 5 tablespoons of the melted coconut oil, the erythritol, the whole egg, the two egg yolks, and the 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 of melted coconut oil in a microwave-safe bowl. Melt on 50% power in 30-second intervals, stirring between intervals, until smooth.
With the mixer running on low, slowly pour the melted chocolate into the egg mixture and continue beating until the mixture is 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 by hand.
Spread the batter evenly into the prepared pan and bake for 20–21 minutes, until the edges darken slightly, the center rises a bit, and a toothpick inserted in the center comes out clean.
Cool the pan on a wire rack for 20 minutes, then use the parchment overhang to lift the brownies from the pan and cool completely on the rack before slicing.