Preheat the oven to 350 degrees F. Line two 9-inch-round cake pans with wax paper or parchment and coat with cooking spray. Prepare the cake mix as the label directs; divide between the pans. Bake as directed.
Let the cakes cool about 15 minutes in the pans, then remove to a rack and cool completely. Using a small serrated knife, trim the top edge of each cake at an angle to give it a rounded shape.
Put 1 cake layer on wax paper or parchment, trimmed-side down. Spread with frosting; top with the other cake, trimmed-side up. Cover the whole cake with frosting, reserving 1 cup. Freeze at least 30 minutes.
Meanwhile, make the sugared grapes: Brush the grapes with the beaten egg white, then spoon the granulated sugar on top. Let dry at least 1 hour at room temperature.
Dust a work surface with confectioners' sugar. Roll out one-quarter of the fondant until about 1/8 inch thick, dusting with more confectioners' sugar as needed.
Cut out an 8-inch circle of fondant and place on a sheet of wax paper or parchment. Center the cake on top.
Add the scraps to the remaining fondant and roll out into a 13-inch circle, about 1/8 inch thick. Drape the fondant over the cake to cover completely.
Smooth the fondant with your hands and tuck it in at the base of the cake; trim the excess with a knife. Cut out a wedge of the cake.
Tint the reserved 1 cup frosting pale yellow with food coloring; spread on the cut sides of the cake. Rub the vegetable oil all over the fondant. Arrange on a boardwith the cookies and grapes.