Preheat the oven to 350 degrees.
Pour margarine into a 9 X 13-inch baking pan. Tilt the pan to swirl the margarine around until the bottom of the pan is completely coated. Sprinkle crumbs evenly into the pan until the bottom of the pan is evenly coated in crumbs. Pat down lightly.
In a medium bowl, whisk together coconut milk, agave, vanilla, salt and cornstarch. Drizzle this mixture evenly over the crumb layer. Sprinkle shredded coconut into the pan, then layer chocolate chips and walnuts. With the palm of your hand, gently pat so that the chocolate chips are firmly pressed into the cookie.
Bake for 25 to 30 minutes, or until center looks thick and slightly bubbly with lightly browned edges. Let cool, and then refrigerate for 8 hours or overnight. Using a sharp knife, cut into small squares.