Preheat oven to 350 degrees F. Lightly coat an 8-inch square baking pan with nonstick vegetable spray.
Bottom Layer: In a dry bowl, set over barely simmering water, melt the butter and bittersweet chocolate. Remove from the heat and stir in the graham cracker crumbs, coconut and walnuts. Pat the mixture into the bottom of the baking pan to form a crust. Bake for 9 minutes until set. Let cool to room temperature, then chill.
Buttercream: Beat the butter and sugar together until light and fluffy, about 2 minutes Slowly beat in the maple syrup and Grand Marnier. Spread over the cooled crust and chill for 20 minutes to set.
Chocolate Glaze: In a dry bowl, set over barely simmering water, melt the chocolate. Remove from heat and stir in the vegetable shortening and corn syrup. Spread the chocolate glaze over the buttercream layer and chill for 5 more minutes. Cut into 2-inch square bars.
