Preheat the oven to 400°F (200°C) and line a baking sheet with parchment paper. Set aside.
To Make the Dough
Peel, core and finely dice the apple. You should have about 1 cup (5 oz/142 g). Place in a bowl and toss with the lemon juice. Set aside.
In a small jug, whisk together the egg, buttermilk and vanilla extract. Set aside.
In a large bowl, stir flour, brown sugar, baking powder, cinnamon, and salt together.
With a pastry blender or your fingers, rub in the butter until the mixture resembles coarse breadcrumbs.
Pour in the buttermilk mixture along with the apples, and stir until the dough comes together.
Shape and Bake
On a floured surface, very briefly knead the dough until no crumbly bits remain, then pat the dough to a 1-inch (3 cm) thick circle (about 7 inches/18 cm wide).
Cut the circle into 8 wedges and arrange on the prepared baking sheet. Brush the tops only with egg wash and sprinkle with cinnamon sugar.
Bake for 22-25 minutes, or until golden brown.
Let cool for at least 10 minutes before cutting and serving with butter.
Store completely cooled leftovers in an airtight container at room temperature for up to 2 days.