Preheat the oven to 170C/325F and line a 20x20cm baking tin with baking parchment.
Combine all of the breakfast bar ingredients in a large bowl. Mix to combine and spoon out into your baking tin. Flatten and squash the mixture down hard (you can use the back of a spoon, but if you want to reduce the chance of crumbling then place a piece of greaseproof paper on top of the flattened mixture, place a same-size baking tin on top and really squash down hard. Then you can peel the paper off) ensuring the mixture reaches all the edges of the tin. Place in the oven and bake for 20-25 minutes until lightly golden on top.
Take out of the oven and leave to cool in the tin for 10 minutes, then lift out of the tin by holding onto the baking parchment. Place on a chopping board on the baking parchment and leave to cool completely before cutting into 12 bars, using a sharp knife. You should be able to slice the bars no problem, but if you do find that the bars crumble when slicing, then place in the fridge for a couple of hours before slicing the rest.
Make the drizzle by whisking the powdered sugar with half of the Greek yoghurt. Add a little more Greek yoghurt until you get a thick sauce. Drizzle this over the breakfast bars and serve.
Leave the drizzle to dry for half an hour or so before wrapping and storing any remaining bars in an air-tight box. These bars are best eaten within 3-4 days.