• 1 tablespoon grapeseed or vegetable oil
• Confectioners' sugar for dusting (about 1 cup)
• 2 envelopes unflavored gelatin
• 1 1/2 cups granulated sugar
• 1/2 cup light corn syrup
• 1 teaspoon vanilla extract

• 5 pounds medium-size garnet yams or sweet potatoes
• 1 1/4 cups graham cracker crumbs
• 1/2 cup toasted walnuts
• 1 tablespoon firmly packed brown sugar
• 1/4 cup melted butter

To make marshmallows:
Brush a 9-inch square pan with oil. Generously dust with confectioners' sugar, tapping out excess. In the large bowl of an electric mixer, pour 1/2 cup water. Sprinkle gelatin on top; let stand until gelatin softens, about 5 minutes.

In a small saucepan over medium-low heat, combine granulated sugar, corn syrup, and 3 1/2 tablespoons water. Bring to a boil, stirring to dissolve sugar. Continue boiling, without stirring, until syrup reaches 240° (soft-ball stage) on a candy thermometer. Immediately remove pan from heat.

Using the whisk attachment of mixer, beat gelatin mixture at high speed. Gradually add hot syrup in a thin, steady stream. Beat 5 minutes longer, until mixture is white, fluffy, and stiff. Beat in vanilla. Pour marshmallow mixture into prepared pan. Using a spatula brushed with oil, spread marshmallow evenly in pan. Let stand uncovered at room temperature 10 to 12 hours.

Dust a large cutting board with confectioners' sugar. Brush a knife with oil, then cut around marshmallow edges to loosen from pan. Cut marshmallow into quarters. Using a spatula brushed with oil, transfer pieces to the cutting board. Cut marshmallow quarters into 1-inch squares.

To make yams:
Preheat oven to 400°. Place yams on a baking sheet; roast 50 to 60 minutes, or until fork-tender. Let cool completely. Peel yams, cut into 1-inch squares, and set aside.

Reduce oven temperature to 350°. In a food processor, combine graham cracker crumbs, walnuts and brown sugar until nuts are finely ground. Transfer to a bowl, add butter and toss with a fork until crumbs are evenly moistened. Press crumbs onto bottom of a 13" x 9" x 2" glass dish. Arrange yams on crust. Top evenly with marshmallows. Bake until marshmallows are melted and browned on top, 25 to 30 minutes.