Preheat oven to 350 °F (175°C). Place a silpat on a baking sheet and set aside.
In a large mixing bowl, whisk together the buckwheat flour, coconut sugar, baking soda, baking powder, and salt. Pour in the water, vanilla extract, and apple cider vinegar and whisk until well combined.
Pour the batter on the silpat and spread it evenly into a thin layer using a spatula. Try to spread it as evenly as possible and quite thin, you don't want it to be paper thin though. It should have a 2-3 mm thickness.
Bake for 12 minutes. Next, remove the baking sheet from the oven and lower the oven temperature to 250°F (125°C).
Let cool for a few minutes. Using your hands, break the dough into 1/2-inch flakes. The dough will be soft; this is normal. Spread the flakes evenly on the silpat and bake for another 25 minutes.
Turn off the oven, slightly open the door and leave it in the oven for another 10 minutes.
Store in an airtight container at room temperature for up to 2 weeks. Enjoy with yogurt, almond milk, or on top of banana ice cream!
Notes
For cinnamon cereals: add 1/4 tsp cinnamon to the batter.
For chocolate cereals: substitute 2 tbsp of buckwheat flour for cocoa powder.
For even crunchier cereals: mix 2 tablespoons of buckwheat flour with 2 tsp water. Mix until it looks like breadcrumbs. If needed, add a bit more water. Sprinkle the breadcrumbs on the batter after you have spread it on the silpat.