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1
Preheat an oven to 325 degrees F (165 degrees C).
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2
Cut a 12x14-inch sheet of aluminum foil. Fold it in half lengthwise, then in half again widthwise.
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3
Wrap the foil partially around a spice jar lengthwise, bringing foil up around the jar and crushing the foil around the ends of the jar to make a boat-shaped baking form about 4 inches long, 2 1/2 inches tall, and 1 1/2 inch wide. Repeat with 15 more pieces of foil to make 16 boat-shaped baking forms. Shape the bottoms of the forms to be slightly flat so forms can stand up.
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4
Spray the forms with cooking spray, and place onto a baking sheet.
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5
Mix together 1 cup water, eggs, and vegetable oil in a bowl with an electric mixer set on low speed.
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6
Beat in yellow cake mix on medium speed for 2 minutes.
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7
Beat in the vanilla pudding mix until smooth, about 3 minutes more.
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8
Divide the batter evenly into the prepared aluminum foil baking forms, filling each form about 1/2 full.
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9
Bake in the preheated oven until a toothpick inserted into the center comes out clean, 11 to 15 minutes. Cool in the forms for 10 minutes before removing to cool completely on a wire rack.
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10
Beat together the butter, shortening, vanilla extract, and salt in a bowl with an electric mixer until smooth, about 3 minutes.
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11
Beat in the confectioners' sugar on low speed, gradually increasing speed to high until frosting is smooth and fluffy.
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12
Mix in the marshmallow fluff, beating on high speed until light and airy.
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13
Spoon frosting into a pastry bag equipped with a round pastry tip.
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14
Remove the cakes from the baking forms.
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15
With a sharp knife, cut a 2-inch slit lengthwise on the flat bottom side of a cake.
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16
Pipe a generous amount of the frosting into the slit, holding the shape of the cake with your hand to prevent splitting. Repeat with the remaining cakes.
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17
For the coconut-raspberry variation: Bring 2 cups water and raspberry gelatin mix to a boil in a saucepan until the gelatin dissolves, about 5 minutes.
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18
Pour mixture into a shallow dish and refrigerate until thickened but not set, about 15 minutes.
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19
Place coconut flakes in a food processor. Pulse until flakes are finely chopped.
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20
Transfer the coconut to a shallow dish and stir in enough cornstarch to achieve a loose texture.
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21
Dip a filled cake in the thickened gelatin, turning to coat all sides; roll the cake in the coconut mixture. Repeat with remaining cakes.
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22
Arrange the coconut-coated cakes on a wire rack until gelatin and coconut are set, about 2 hours.
