In a large bowl, add flour, salt, and cinnamon or other spice (if using). Whisk together well to distribute the salt and cinnamon with the flour. Set aside.
In a separate bowl, mix together the yogurt and milk (or other liquid being used) and add in vanilla extract or another flavored extract, and sweetener, if using. Stir well.
Pour the liquid into the dry ingredients. Mix well until the flour mixture is fully incorporated with the liquid and a dough has formed.
Cover the bowl with the dough in it and set aside for 8-12 hours in a warm place out of direct sunlight. If you are not comfortable leaving this dough on your counter, you may refrigerate it, but increase the "soak" time to 18 hours. If you are using sprouted flour, you can skip this soak time and proceed directly with the next step.
After the soak time is complete, pour the melted fat over your dough and work it into the dough until it is completely incorporated. Set aside.
Preheat the oven to 300°F.
Next, prepare 2-3 half-sized baking sheets (18" x 13"). If you are not using parchment paper to roll out your dough, then grease each baking sheet with whatever fat you are using in your dough mixture. Set aside.
You may also use waxed paper for the rolling process. However, waxed paper can NOT go into the oven, so both the top and bottom sheet of waxed paper would need to be removed when you transfer the rolled dough to the baking sheet. And you will want to make sure that you are transferring your dough to a greased baking sheet.
If using parchment paper to roll out your dough, place the parchment paper on a flat surface.
Divide the dough into two pieces. Place one piece of dough on the prepared parchment paper, and cover the dough with a second piece of parchment paper. If you are not using parchment paper, place the piece of dough directly on your greased baking sheet.
Roll out the dough either between the parchment paper or directly on the baking sheet until it is approximately 1/8" thick or less. If you are rolling out the dough between the parchment paper, you will want to roll it to the approximate size of your baking sheet. If you are rolling out the dough directly on your baking sheet, grease your rolling pin. (You may also use your hands to pat out the dough to the desired thickness, but make sure you grease your hands.)
If at any time, some dough oozes out between the parchment paper or seems too thick to be rolled out thinly on the baking sheet, remove some of the dough and return it to the bowl of dough. If this happens to you, you will most likely need to divide the dough into three portions and use three baking sheets.
If using parchment paper, once you have rolled the dough to the desired thickness, remove the top sheet of parchment paper from the dough. If the parchment paper sticks, dip your fingers in a bit of water and use your moistened fingers to help release the parchment paper from the dough.
Transfer the dough, using the lower piece of parchment paper, to the baking sheet.
Place the baking sheet on the middle rack in the preheated oven and bake until the dough is golden brown and crisp, approximately 45 minutes to 1 hour or more. The baking time will depend greatly on how thin you have rolled out your dough and may take more than 1 hour.
If you are baking two baking sheets of dough at one time on different racks in your oven, rotate the baking sheets from one rack to the other halfway through the baking time.
Once the dough is fully baked, remove the baking sheets from the oven and allow the baked dough to cool on the baking sheets. The dough should now be fully baked and quite hard to the touch and crisp.
Once the baked dough is completely cooled, you can break the baked dough into small pieces by hand. (See video.) You can also cover the baked dough with parchment paper or waxed paper and roll over it with a rolling pin a few times until the baked dough is broken into a lot of small pieces.
Once the dough—which is now your homemade flaked cereal—is completely broken into small pieces, you will want to store it in an airtight container. This cereal is shelf-stable, unrefrigerated, and you can keep it in your pantry. It will stay fresh and crisp for 3-4 weeks.
When you are ready to eat the cereal, you can put it in a bowl with milk just as you would a boxed flaked cereal from the grocery store. You can also eat your flaked cereal dry as a snack.