Brown the butter in a small saucepan over medium heat, swirling often, until it turns golden brown and smells nutty, about 5 minutes. Watch it closely so it doesn't burn. Pour into a large mixing bowl.
Add the maple syrup, sourdough starter, vanilla extract and salt to the browned butter. Whisk until combined.
Add the oats, chopped nuts and puffed buckwheat. Stir until every oat is coated.
Preheat the oven to 160°C (320°F). Divide the mixture between two parchment lined baking sheets and press into a thin, even layer with a spatula.
Bake for about 40 minutes, flipping the granola with a spatula every 15 to 20 minutes so it browns evenly. It's done when golden brown, it will still feel a little soft, that's normal.
Let the granola cool completely on the baking sheets. This is what makes it crunchy.
Once fully cool, fold in the chopped dark chocolate.
Store in an airtight glass container.
