Preheat the oven to 220C/425F/gas 7. Mix the flour, baking powder, xylitol or Natvia, poppy seeds and salt. Rub in the butter with your fingers until mostly incorporated, then add yogurt, lemon zest and vanilla extract and bring together to form a dough (alternatively, pulse the butter into the dry ingredients in a food processor for 30 secs, then add the rest of the ingredients and pulse until it starts to form a dough).

Flatten the dough into a large circle (don't worry too much about thickness, but probably 2-3cm is good) on a lined baking tray. Cut gently into 8 slices with a sharp knife, but leave intact as a circle (it's fine if the knife goes all the way through - just don't break the triangles up). Brush with a little extra melted butter, milk, or a beaten egg. Bake for 12-15 mins. Best served warm.