Preheat oven to 375 degrees. Line baking sheets with parchment paper or a non-stick baking mat. In a mixing bowl, combine graham cracker crumbs, flour, cocoa powder and baking powder. Set aside. In a large mixing bowl beat together condensed milk and butter until mixture is creamy. Stir in graham mixture; mix well. Stir in white chocolate and pecans. Refrigerate 30 minutes. Drop by large tablespoonfuls on prepared baking sheets. Bake 12 to 14 minutes or until firm. Transfer to a wire rack to cool completely. When cool, store in an airtight container.