Preheat your oven to 350°F (175°C) and line an 8x8 inch baking pan with parchment paper.
In a bowl, mix together the almond flour, cocoa powder, baking powder, and salt until well combined.
In a separate microwave-safe bowl, melt the dark chocolate in short intervals, stirring in between until smooth.
Stir in the melted butter, melted dark chocolate, granulated sweetener, egg, and vanilla extract until the batter is smooth.
Fold in the chopped walnuts (optional).
Pour the batter into the prepared pan and spread it evenly.
Bake the brownies for 15-18 minutes, or until a toothpick inserted in the center comes out clean.
Let the brownies cool to room temperature, and then chill in the refrigerator for at least 30 minutes.
Cut the brownies into squares and serve chilled.