Preheat the oven to 350 F.: The moment you preheat you start the transformation of moisture to crispness, and you can already smell the faint warm, toasty note from the empty oven as it climbs. A properly preheated oven ensures the crackers begin setting on contact with heat, which helps create an even golden color, rather than slowly drying and becoming tough. A common mistake is placing the crackers into an oven that is not hot enough, which can lead to a limp or dense texture, so always wait until the temperature reaches 350 F. If your oven runs cool, allow a few extra minutes, and consider using an oven thermometer to confirm temperature.
In a large bowl, combine the flours, salt, garlic powder, and cheese.: As you mix these dry ingredients, notice the change in aroma when the cheddar cheese meets the flours, giving off a nutty, slightly tangy scent. The tactile contrast between the slightly gritty brown rice flour and the silky tapioca flour is important because it influences how the dough will hydrate. Mixing the seasoning now ensures even distribution so every cracker is seasoned equally. Avoid over mixing to prevent the cheese from clumping; if the cheese lumps form, break them apart with your fingers so they are dispersed.
Add the melted butter and water 1 tablespoon at a time mixing to combine after each addition.: Introducing melted butter and water gradually lets you control the dough consistency, so you can stop as soon as the mixture holds together without becoming sticky. The butter adds a rich mouthfeel and helps with browning, and the water activates the flours so they knit into a cohesive dough. One pitfall is adding all the water at once, which can make the dough too wet and tough to roll, so add one tablespoon and mix, then decide if more is needed.
Once a dough forms, knead it into a ball.: The knead should be gentle and brief, just enough to bring the dough into a smooth ball, and you will notice the dough becoming more uniform and slightly tacky. Kneading aligns the little bits of cheddar cheese and helps even hydration, which matters for an even bake. If you knead too vigorously, especially with tapioca present, the dough can become gummy, so stop once the surface looks coherent.
Place the dough on parchment paper or a nonstick rolling mat. Roll out into a rectangular shape until the dough is about ?-inch thick.: Rolling will spread the dough into an even layer, and when it is the right thickness you can hear a faint papery sound as the rolling pin glides. The suggested thickness produces crackers that crisp up in the oven while still holding shape. Uneven rolling causes thin edges to brown faster than the center, so check the thickness visually and by touch. A typical issue is making the dough too thin near the edges, so roll from the center outwards to keep it even.
Slice into crackers with a knife, pizza slicer, or cookie cutters (you can re-roll scraps and rough edges to make more.): Cutting reveals the true texture of the rolled dough, and using a sharp pizza cutter or knife gives clean edges that bake evenly. Re rolling scraps is economical, but be mindful that repeated rolling can toughen the dough, so combine scraps sparingly. If your crackers crumble when cutting, the dough may be too dry, so lightly press the pieces back together before recutting.
Transfer the parchment paper with the crackers to a baking sheet. For a more golden look, lightly spray or brush the tops of the crackers with melted butter or oil. Sprinkle extra sea salt and cheese over the top, if desired.: Brushing with a little butter encourages browning and gives the surface a subtle sheen, which enhances both look and flavor as it heats. The extra sprinkle of cheese and sea salt crisps and toasts, delivering a pleasing crunch and visual appeal. Avoid drenching the tops with too much fat, which can make the crackers greasy instead of crisp; a light brush is all it needs.
Bake for about 12 minutes, until starting to turn golden. Turn the oven off, but leave the crackers in the oven for 5 minutes. Remove from the oven and let cool completely. Store in an airtight container.: As they bake you will see the edges take on a warm, golden hue and a roasted, cheesy aroma will fill the kitchen. Turning the oven off and leaving the crackers inside lets residual heat continue crisping without over browning, producing a more reliable texture. A typical error is removing the crackers as soon as the timer dings; if they look slightly pale, the residual heat step will finish them. Cool completely to avoid steam softening them before storage.