Preheat oven to 400°.
Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and next 6 ingredients (through cinnamon) in a food processor; pulse until mixture resembles coarse meal.
Combine banana and brown sugar in a medium bowl; let stand 5 minutes. Add flour mixture and 1/4 cup buttermilk alternately to banana mixture, stirring just until moist.
Turn dough out onto a lightly floured surface; knead lightly 1 1/2 minutes with floured hands. Pat dough into a 6-inch circle on a baking sheet lined with parchment paper. Cut dough into 8 wedges, cutting into, but not through, dough. Brush 1 1/2 teaspoons buttermilk over surface of dough; sprinkle with granulated sugar. Bake at 400° for 12 minutes or until lightly browned. Remove from pan; cool on wire racks.