1/2 cup (1 stick) unsalted butter, softened
1/4 cup brown sugar
1/2 cup granulated sugar
1 large egg
2 teaspoons vanilla extract
3 medium overripe bananas, mashed
1/4 cup plain or vanilla yogurt
1 cup all-purpose flour
1/2 cup cocoa powder
1 teaspoon baking soda
1/2 teaspoon salt
1 cup semisweet chocolate chips