Preheat oven to 375F and line a square baking pan with parchment paper.
Mash the vegan butter (or butter of choice) and both sugars together until smooth. Mash in the banana and then the egg, mixing until completely smooth.
Add the salt, flour, cocoa, and baking powder and mix well. But do not overmix or the batter will get tough. Fold in the chocolate chunks.
Spoon half the batter into your lined baking tray and smooth it out. Spread the sunflower seed butter in an even layer over the batter. Then spoon the rest of the batter on top for the final layer and level off the top with a butter knife.
Place in the preheated oven and bake for about 35 minutes, until a toothpick inserted into the centre comes out relatively clean.
Allow the bars to cool and then cut into squares.