Surprise Sweet Potato Candy

In a large saucepan combine sugar, margarine, and milk and cook over medium heat, stirring frequently, until mixture boils. Add sweet potatoes. While stirring, continue to boil until mixture reaches soft-ball stage (235 degrees on a candy thermometer). Remove from heat and add marshmallows and graham cracker crumbs. Stir until marshmallows are melted and well blended. Add vanilla and pecans, mixing well. Pour into a buttered 9-inch square baking dish. Cool completely before cutting into squares.