I absolutely love cereal and it can be really fun to make your own — plus it’s a great project with kids because they can flavor the flakes exactly the way they like. When spreading out the batter, remember that the thinner you spread, the crunchier and flakier the flakes will be!
Preheat the oven to 350 degrees F. Line a rimmed baking sheet with parchment paper and lightly spray with oil.
Whisk together the whole-wheat flour, wheat germ, cinnamon and salt in a medium bowl. Mix the honey and vanilla with the water in a liquid measuring cup, then slowly pour the liquid in a stream into the flour mixture, stirring constantly, until it becomes a pourable batter (like a thin pancake batter); add more or less water as necessary. Pour the batter onto the prepared baking sheet, then use an offset spatula to spread the batter evenly all the way to the edges, smoothing the top so that it is quite thin (you can also use a rolling pin and roll the batter out between two sheets of parchment paper). It should be no more than 1/4 inch thick.
Bake until the top is dry and begins to crack, 15 to 18 minutes. Let cool until it is cool enough to handle, 10 to 15 minutes. Reduce the oven temperature to 250 degrees F.
Break the baked layer into 1/2- to 1-inch pieces. Spread the broken pieces evenly on the same baking sheet. Bake, stirring halfway through and redistributing on the baking sheet, until the flakes are dried and crisp, 45 to 50 minutes. Allow to cool completely. The cereal can be stored in an airtight container at room temperature for up to 2 weeks.