Banana Bread Scones

1 extra-large, extra-ripe banana, mashed (1/2 cup)
1/3 cup heavy cream, plus more for brushing
1 large egg, lightly beaten
1 teaspoon pure vanilla extract
2 cups all-purpose flour
1/3 cup packed dark brown sugar
2 1/2 teaspoons baking powder
1/2 teaspoon kosher salt
1/2 cup (1 stick) cold unsalted butter, cut into pieces
2 1/4 ounces bittersweet or semisweet chocolate, chopped (about 1/2 cup)
1 1/4 ounces (about 1/2 cup) hazelnuts, toasted, skinned, and finely chopped (see Cook's Note)
Sanding sugar for sprinkling

Preheat the oven to 425 degrees F.
In a small bowl, whisk together the banana, cream, egg, and vanilla. In a large bowl, whisk together the flour, brown sugar, baking powder, and salt.
Cut in the butter with a pastry blender, or 2 knives used like scissors, until the mixture resembles coarse meal with some larger pieces. (You can use your fingers if you work fast enough that the butter stays cold.) Toss in the chopped chocolate and hazelnuts.
Drizzle in the banana mixture and stir with a fork until the mixture is crumbly and moistened, but stop before you've mixed it into a uniform batter. Tip out the mixture onto a parchment-lined baking sheet and pat it into a 6-inch circle. Using a sharp knife or a bench scraper, cut the circle into 8 equal triangles. (At this point, you could freeze the triangles well-wrapped on their sheet for another day. Bake them from frozen.) Spread the triangles apart and brush the tops with the cream. Sprinkle with sanding sugar.
Bake until the scones are golden brown and set, 22 to 25 minutes. A toothpick inserted into the center of a scone should come out clean. Transfer the baking sheet to a wire rack to cool slightly. Serve warm or at room temperature. Scones are best the day they're made.