Cut a 12x14-inch sheet of aluminum foil. Fold it in half lengthwise, then in half again widthwise.
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.
Spray the forms with cooking spray, and place onto a baking sheet.
Mix together 1 cup water, eggs, and vegetable oil in a bowl with an electric mixer set on low speed.
Beat in yellow cake mix on medium speed for 2 minutes.
Beat in the vanilla pudding mix until smooth, about 3 minutes more.
Divide the batter evenly into the prepared aluminum foil baking forms, filling each form about 1/2 full.
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.
Beat together the butter, shortening, vanilla extract, and salt in a bowl with an electric mixer until smooth, about 3 minutes.
Beat in the confectioners' sugar on low speed, gradually increasing speed to high until frosting is smooth and fluffy.
Mix in the marshmallow fluff, beating on high speed until light and airy.
Spoon frosting into a pastry bag equipped with a round pastry tip.
Remove the cakes from the baking forms.
With a sharp knife, cut a 2-inch slit lengthwise on the flat bottom side of a cake.
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.
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.
Pour mixture into a shallow dish and refrigerate until thickened but not set, about 15 minutes.
Place coconut flakes in a food processor. Pulse until flakes are finely chopped.
Transfer the coconut to a shallow dish and stir in enough cornstarch to achieve a loose texture.
Dip a filled cake in the thickened gelatin, turning to coat all sides; roll the cake in the coconut mixture. Repeat with remaining cakes.
Arrange the coconut-coated cakes on a wire rack until gelatin and coconut are set, about 2 hours.