Preheat oven to 400 degrees F. Line a baking sheet with parchment paper; set aside.
In a large bowl, whisk together the flour, oats, brown sugar, baking powder, baking soda and salt. Add the butter and use a pastry blender to cut the butter into the flour mixture until the butter is pea-size (alternately, you could use your fingertips to rub the butter into the flour mixture).
In a small bowl, whisk together the buttermilk and the egg yolk.
Make a well in the flour mixture and pour the buttermilk mixture into the center of the well, then add the peanut butter. Using a rubber spatula, very gently fold the mixture together until it starts to come together. Add the chocolate chips and gently knead with your hands (in the bowl) until just incorporated. Be careful not to overmix the dough.
Turn the dough out onto the parchment-lined baking sheet and pat it into a round disk about 8 inches in diameter. Whisk the egg white, then brush the top of the dough with the egg white, and sprinkle with the additional sugar.
Cut the dough into 8 wedges (do not separate them) and bake for 18 to 22 minutes, or until the scones are golden brown and a toothpick inserted in the center comes out clean or with just a few crumbs attached. Remove from the oven and let cool for 5 minutes, then re-slice and separate them. Transfer to a wire rack to cool completely, or serve slightly warm. Store in an airtight container at room temperature for up to 2 days.