Preheat the oven to 350 degrees. Combine the first 5 ingredients in a bowl.  Cut in the butter until the size of peas.  Add the yogurt or sour cream, egg yolk, bananas and chocolate chips. Combine into a ball.  Flour a surface and knead gently until it comes together.  Grease a cookie sheet.  Place ball in center of sheet and pat down to form a disk, about 2-1/2 inches high. Cut dough in half and then in half the other way (now you have 4 pieces). Now repeat again. So now you have 8 triangles.  Pull triangles out from each other so they are not touching too much.  This will give you lots of nice crust.  Bake for about 20 minutes.  Time depends on how thick you made the scones.  To test for doneness - check the centers, they should be firm.You can also make smaller scones by forming dough into two balls and cutting into triangles.