Line a shallow baking tin with parchment paper and preheat the oven to 170°C. We used a 24 x 24cm (about 10 inch) square oven dish.
Place all the ingredients for the oat base in a mixer and blend well. These are the sunflower seeds, rolled oats, oat flour, melted butter, salt, coconut sugar and cinnamon. Every minute pause the blender to scoop the mixture off of the sides. That way the mixture is fully blended.
You should have a clumpy mixture. If needed add up to 3 tablespoons of water gradually to make it stickier.
Spoon your mixture into your oven dish and press down on it firmly to make a thick crust that is evenly distributed. It should be about 1 cm depth.
Prick with a fork and bake on the middle shelf of the oven for 10-15 minutes, or until the edges are golden.
Remove and let the base cool completely. This step is important to stop the base from being too crumbly.
Prepare the chocolate ganache. In a heatproof bowl place the milk chocolate in pieces, the double cream and the sugar. Set the bowl over a saucepan of boiling water. Keep stirring until the milk chocolate is all melted.
Pour the chocolate ganache over the crumble base and spread it out evenly.
Add the marshmallows on top by standing them upright over the chocolate layer. (Kids love to help with this part!)
Place the dish in the oven with a top heat (or grill) for a few minutes until the marshmallows are browned. Keep a close eye on it as it cooks as the marshmallows can burn quickly.
Remove and cut into bars or squares ready to serve.