Preheat oven to 350 degrees F (175 degrees C). Grease a 9-inch baking pan.
Spread mixed nuts onto a baking sheet.
Toast in preheated oven until nuts start to turn golden brown and become fragrant, 10 to 12 minutes. Let cool.
Mix brown sugar, flaxseed oil, and butter together in a bowl until creamy. Add eggs, 1 at a time, whisking well after each addition. Stir in vanilla extract. Add 2/3 cup toasted nuts, almond flour, cocoa powder, all-purpose flour, dark chocolate, and baking powder; mix batter well.
Pour batter into the prepared baking pan. Sprinkle remaining 1/3 cup toasted nuts on top.
Bake in the preheated oven until a toothpick inserted into the center comes out clean, about 20 minutes.
