Preheat the oven to 180°C or 350°F. Grease a 9x5 inch loaf pan generously with butter or margarine.
Place the chocolate in a microwave safe bowl. Cook on high power stopping to stir in between, until about 80% melted. Stir until smooth, letting the residual heat in the bowl melt the remaining bits of chocolate. Add the cocoa powder and stir until dissolved. Set aside.
Place sugar and butter or margarine in a large bowl, and beat with a mixer at medium speed until well blended. Add the eggs, 1 at a time, beating well after each addition. Add mashed banana, yogurt, and vanilla; beat until blended. Add flour mixture; beat at low speed just until blended.
Roughly divide the banana bread batter into two. Add the melted chocolate to one batter and stir until well combined.
Spoon the plain batter and chocolate batter alternately into the prepared loaf pan. Swirl the batters together with a knife (do not overdo it or the colors will get muddled up). Bake for about 50-60 minutes, or until a toothpick comes out clean. Cool in the pan on a wire rack for about 10 minutes, then remove the loaf from the pan and set on the wire rack to cool completely. Store banana bread at room temperature for up to 3 days, in the refrigerator for a week and in the freezer for 3 months.