Bake until planks are firm to the touch, 20-25 minutes. Transfer the planks, still on parchment paper, to a cooling rack and let cool 5 min.
Reduce oven temp to 250.
Transfer planks to a cutting board, use a pastry brush to dust off any excess flour.
With a large serrated knife, cut each plank into 18 slices. Arrange slices on a baking sheet and return to 250 degree oven, baking until slices are dry & firm, about 1 hour.
Allow to cool completely.