Melt butter in a medium saucepan over medium heat. Remove from heat, add in brown sugar and cocoa powder and whisk until well combine. While whisking vigorously, slowly pour in beaten egg. Return mixture to heat and cook for 1 - 2 minutes until mixture has thickened. Remove from heat, stir in vanilla. Add in graham cracker crumbs, coconut, and pecans and toss until evenly coated. Press mixture into a buttered 8 by 8-inch or 9 by 9-inch baking dish. Cover with plastic wrap, place in freezer for 20 minutes or refrigerate for 40 minutes.