Preheat oven to 350°. Grease or use non-stick spray on 9x5 pan

Lightly spoon flour into dry measuring cups and level with a knife.

Combine the flour, baking soda, and salt, stirring with a whisk.

Place sugar and butter in a large bowl, using mixer, beat at medium speed until well blended. Add the eggs, 1 at a time, beating well after each addition.

Add mashed banana, yogurt, peanut butter and vanilla and beat until blended.

Add flour mixture and beat at low speed just until moist.

Measure out 1 cup of batter and add 1/2 cup melted semisweet chocolate chips to batter and using a spoon or whisk mix together in a small bowl until just combined and .

Spoon plain batter into pan, alternating with dollops of chocolate batter. Drag a knife through batter to swirl. Bake as directed.

Bake at 350° for 1 hour or until a wooden pick inserted in center comes out clean.

Cool 10 minutes in pan on a wire rack and remove from pan. Cool completely on wire rack.