Make the filling: Combine the grapes, apple and water in a medium Dutch oven over medium-low heat. Cover and cook until most of the grapes pop and shrivel, about 12 minutes. Increase the heat to medium and stir in the granulated sugar. Cook, stirring often and mashing with a potato masher occasionally to crush the apple, until the filling is thick and syrupy, 15 to 20 minutes. Remove from the heat and let cool completely.
Make the cake: Preheat the oven to 350 degrees F. Butter and flour three 8-inch round cake pans; line the bottoms with parchment paper. Sift the flour, baking soda and salt into a medium bowl. Beat the granulated sugar and vegetable oil in a large bowl with a mixer on medium-high speed until it looks like wet sand, about 2 minutes. Scrape down the bowl and add the eggs, one at a time, beating well after each addition. Add the vanilla; beat on high speed until light, about 2 minutes. Reduce the mixer speed to low; beat in the flour mixture in three batches, alternating with the buttermilk, beginning and ending with the flour. Continue beating until smooth, about 1 minute.
Divide the batter among the prepared pans and bake until a toothpick inserted into the centers comes out clean, 25 to 30 minutes. Let cool 10 minutes in the pans, then invert onto racks; remove the parchment and let cool completely.
Meanwhile, make the topping: Line a rimmed baking sheet with parchment paper. Whisk the egg whites in a medium bowl until just foamy. Spread the granulated sugar in a pie plate or shallow bowl. Dip the grapes in the egg whites, letting any excess drip back into the bowl, then roll in the sugar. Transfer the sugared grapes to the prepared baking sheet. Freeze at least 30 minutes.
Make the frosting: Beat the butter and salt in a large bowl with a mixer on medium speed until smooth and fluffy, about 2 minutes. Gradually beat in the confectioners' sugar, then the vanilla. Increase the mixer speed to medium high and beat until very light and fluffy, about 2 more minutes.
Assemble the cake: Put 1 cake layer on a cake stand or serving plate. Spread about one-third of the grape filling on the cake, then top with another cake layer and half of the remaining filling. Top with the third cake layer. Cover the top and side of the cake with the frosting. Refrigerate to set the frosting, 15 to 20 minutes. Spread the remaining filling on top of the cake, leaving a 1/2-inch border. Arrange the frozen grapes around the border.
Photograph by David Malosh
*If you can't find the seedless variety, you'll need to strain the filling at the end of Step 1 to remove the seeds. Halve and seed the grapes for garnish.