Preparation

• Preheat the oven to 350°F (180°C) and butter and line with parchment an 8x8-inch (20x20cm) baking pan. Set aside.
• In a medium mixing bowl, combine the flour, baking powder, and salt.
• In a separate mixing bowl, whisk together the butter, sugar, eggs, and vanilla extract.
• Combine the wet and dry ingredients and then divide the batter evenly into 2 separate bowls.
• In one bowl, sift in the cocoa powder and stir in the chopped chocolate.
• In the other bowl, stir in the mashed banana.
• Dollop the brownie batter into the prepared pan, then pour over the banana batter.
• Take a butter knife and swirl the chocolate batter through the banana batter without mixing it completely, to create a marbled look.
• Bake for 40-45 minutes, until a wooden skewer inserted in the middle comes out clean. Let cool completely before cutting into squares.
• Store in an airtight container at room temperature for up to 3 days, or freeze for up to 2 months. Defrost at room temperature or in the microwave for a minute.