Preheat oven to 175C(347F) fan-forced and line a baking tray with baking paper.

In a large bowl, whisk the flour, baking powder, baking soda, salt and sugar together.

Add the butter and cut it into the dry ingredients until it resembles corse bread crumbs (about pea size).

In another bowl, combine the yoghurt, egg and vanilla and then gently stir it into the butter/flour mixture.

Turn it all out onto a lightly floured surface and knead it gently just until it comes together.

Cut the dough into two equal pieces and shape them into discs about 18cm in diameter.

Place one of them on the lined baking tray.

In a small bowl, mix the flour, butter, cinnamon and brown sugar together to make the crumbly streusel.

Sprinkle it over the top of the dough dish on the tray leaving a little border.

Place the second dough disc on top of the first and gently pinch the edges together.

Use a sharp knife to slice it up like a pizza into 8 wedges but don't separate them.

Brush the top of the dough with egg white and finish by sprinkling with the caster sugar.

Brush with egg white and sprinkle with sugar.

Bake the giant scone for 20-25 minutes or until it is golden brown.

Leave it to rest on the tray for 5 minutes and then transfer to a wire rack to cool slightly.

Serve warm or at room temperature.