Preheat your waffle maker. Combine all the dry ingredients in a medium bowl and the melted butter, milk, sweet potato and egg yolk in a separate large bowl. Gradually add the dry ingredients to the wet mixture and stir together until smooth.
Beat the egg whites until stiff. Gently fold into the batter in 3 parts until combined. The batter should be thick but still able to move. Lightly brush your waffle maker with the oil. Using a ladle, pour in enough batter to fill and then bake for 8 minutes. They should be risen and golden brown. Remove and keep warm in a low temp oven until needed.
Preheat the oven to 200°C/gas mark 6. Combine the 2½ tbsp sugar and rosemary leaves in a bowl, then add the apples, tossing together until coated. Spread in a single layer on a medium rimmed baking sheet and then bake, turning halfway through, for 20-25 minutes until tender but still retaining bite.
As the apples bake, start on your cider syrup. Add the cider, 200g light brown sugar, vanilla extract and cinnamon sticks along with a pinch of sea salt to a medium saucepan. Cook on low until the sugar has dissolved. Bring to a gentle boil for 10-15 minutes until the mixture has reduced to about half and thickened. Remove from the heat and leave to cool slightly.
Serve the waffles piled with the roasted apples and warm cider syrup and a dollop of Greek yogurt.