SLICE BREAD: If using French bread, slice into 3/4th inch thick pieces. Slice each piece into 2-3 thick "sticks." If using cut bakery brioche bread, cut each piece into 4 "sticks." Set aside (uncovered) while preparing everything else.
WET INGREDIENTS: In a large bowl, add the 4 large eggs, 2/3 cup whole milk, 2 tablespoons brown sugar, 1 tablespoon vanilla extract, 1/4 teaspoon fine sea salt. Whisk until completely smooth.
CRUSH CEREAL: In a blender or food processor, add the 6 cups measured cereal (you may need to do this in batches depending on blender/processor size). Pulse cereal until it forms fine crumbs and then transfer that crushed cereal to a large, shallow bowl.
COAT BREAD: Working with 1 stick of bread at a time, dip in the egg and milk mixture, gently shaking off excess (dip quickly so the tender brioche bread doesn't break), and then very generously coat/dredge with cereal crumbs. Repeat until all of the bread is coated. You may have a bit extra of the egg coating and cereal depending on the size of bread being used and how well coated the sticks are. (See Note 3)
COOK: Place a large nonstick skillet over medium-low heat (or heat an electric griddle to 375 degrees F) and add a pat of butter (or run a stick of butter across the heated griddle) Once the butter has melted and is lightly bubbling, add French Toast Sticks into a single layer in the pan, 1/2-inch apart from one another. Don't over-crowd; make sure the sticks have plenty of room. Cook the sticks until they are crispy and golden brown and then flip to the other side. Once that side is crispy, get the sides and then transfer to a wire cooling rack (See Note 4). Wipe out the skillet or wipe off the griddle with a paper towel and repeat with the remaining sticks until all are cooked.
SERVE: Serve French Toast Sticks immediately off the griddle/skillet with a dust of powdered sugar, fresh berries, and maple syrup (or vanilla sauce) if desired.