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.