Heat oven to 350°F. Prepare a rimmed baking sheet with non-stick spray or oil. Combine flour, salt, and baking soda in a large bowl and whisk to combine. Add buttermilk and barley malt syrup to the bowl and mix just until all dry ingredients are evenly combined.
Scrape batter out onto prepared sheet pan and smooth out to the edges as evenly as possible. Bake until edges are just browning and pulling away from the pan, about 20 minutes.
Loosen cake with a spatula and flip out on a cooling rack immediately. Reduce oven to 275°F. Set aside to cool about 40 minutes.
Working in four or five batches, roughly break up cake into chunks by hand and then pulse in food processor until bits reach desired size.
Spread cereal bits across two rimmed baking sheets and bake until completely dry (about 45 minutes), stirring cereal and rotating pans every 15 minutes.
Once dry, turn off oven, crack door, and leave to cool. Store in air-tight container.