Preheat the oven to 375 degrees F. Coat an 8-by-8-inch baking pan with cooking spray.
Combine the graham crackers, coconut, walnuts, condensed milk, chocolate chips and salt in a medium bowl. Stir with a wooden spoon until thoroughly combined. Transfer the mixture to the prepared pan and press to the edges.
Bake until the chocolate melts and the top is golden, 15 to 20 minutes. Stir the hot mixture in the pan until the chocolate is incorporated throughout. Press back out to the edges and set aside until cool enough to handle.
Spread the confectioners' sugar on a dinner plate. Scoop out the brownie mixture by the tablespoon and shape into balls. While still warm, roll each ball in the sugar to coat. The bonbons will keep, tightly covered, for 2 days.