Preheat the oven to 350ºF. Place the Nature Valley granola bars in a food processor and process to a coarse crumb consistency. In a large mixing bowl, add the granola bar crumbs, flour, brown sugar and melted butter and stir to mix together. Spread the mixture out onto a pan lined with parchment paper and bake for 20 minutes, stirring occasionally. Let cool completely. Spread half of the crumb mixture onto the bottom of an 8x8-inch baking dish. Reserve the other half of the crumbs.