Preheat the oven to 180C and line a baking tray with baking paper.
Combine the oats, buckwheat, cashews, and coconut sugar in a mixing bowl. Add in the coconut oil and maple syrup and mix well making sure everything is evenly coated.
Spread it into a thin-medium layer – about 1cmthick on your lined baking tray. Using the back of a wooden spoon press it down. You want the granola to be packed and stuck together.
Bake for 10 minutes, turn the pan around, and bake for another 10 minutes.
Remove and let cool completely – do not touch it, it needs to set completely so it breaks into large chunks.
Once the granola bark is completely cool, melt the chocolate and spread it over the bark in an even layer. If using, sprinkle fleur de sel over the chocolate.
Place in the fridge/freezer to set completely. Once the chocolate is fully set, break it into pieces.
Store in an airtight container at room temperature for up to a week.