Preheat the Oven: Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper.
Mix Dry Ingredients: In a large bowl, whisk together the flour, sugar, baking powder, baking soda, salt, and cinnamon.
Cut in the Butter: Add the cold butter cubes to the flour mixture. Using a pastry cutter or your fingers, work the butter into the flour until the mixture resembles coarse crumbs with pea-sized pieces of butter.
Prepare Wet Ingredients: In a separate small bowl, whisk together the mashed banana, buttermilk, egg, and vanilla extract until smooth.
Combine Wet and Dry Ingredients: Make a well in the center of the dry ingredients and pour in the wet mixture. Stir gently until the dough begins to come together (it will be a bit sticky). Be careful not to overmix.
Fold in Nuts and Sea Buckthorn Berries: Add the chopped nuts and sea buckthorn berries and fold them into the dough until evenly distributed.
Shape the Dough: Turn the dough out onto a lightly floured surface. Gently pat it into a round disc about 1 inch thick. If the dough is too sticky, lightly flour your hands or the surface. Use a knife or bench scraper to cut the disc into 8 wedges.
Prepare for Baking: Transfer the wedges to the prepared baking sheet, leaving some space between them. Brush the tops with buttermilk and sprinkle with a bit of sugar for extra crunch and sweetness.
Bake: Bake for 15-18 minutes, or until the scones are golden brown and a toothpick inserted into the center comes out clean.
Cool: Let the scones cool on the baking sheet for about 5 minutes, then transfer them to a wire rack to cool completely. Enjoy them warm or at room temperature.