Make the cake: Preheat the oven to 350˚. Butter the bottom and sides of two 9-inch round cake pans. Dust with flour and tap out the excess; set aside. Whisk the flour, baking powder, salt and baking soda in a medium bowl.
Beat the butter and brown sugar in a large bowl with a mixer on medium-high speed, scraping down the bowl as needed, until very light and fluffy, 5 to 7 minutes. Beat in the eggs, one at a time, until combined; continue beating until well blended, 1 to 2 more minutes. Reduce the speed to medium low; beat in the flour mixture in three batches, alternating with the buttermilk and vanilla in two batches, until just smooth (it’s OK if the batter looks curdled at first—it will smooth out). Stir the batter a few times with a rubber spatula to make sure it’s blended.
Divide the batter between the prepared pans. Bake until the cakes are browned, the edges begin to pull away from the pans and the centers spring back when pressed, 35 to 45 minutes. Transfer to a rack and let cool 10 minutes in the pans, then remove the cakes to the rack to cool completely.
While the cakes cool, make the crumble: Toss the potato chips, cookies, coconut, melted butter and brown sugar in a medium bowl until well coated. Spread on a baking sheet and bake until lightly browned, about 10 minutes. Let cool, then stir in the chocolate-covered raisins and/or nuts; set aside.
Make the frosting: Mix the brown sugar and 2 tablespoons heavy cream in a small microwave-safe bowl; microwave, stirring once or twice, until smooth, 45 seconds to 1 minute. Let cool slightly. Beat the butter and salt in a large bowl with a mixer on medium speed until creamy, about 2 minutes. Beat in the confectioners' sugar in two batches until smooth and fluffy. Add the brown sugar mixture; beat on medium-high speed until fluffy and spreadable, 1 to 2 more minutes.
Make the ganache: Combine the remaining 1/3 cup heavy cream and the chopped chocolate in a microwave-safe measuring cup or bowl; microwave, stirring every 30 seconds, until melted, about 1 minute. Let the ganache cool until it’s thickened but still spreadable.
Put 1 cake layer on a cake stand or serving plate. Top with 1 cup frosting, spreading it to the edges. Sprinkle with a heaping 1/2 cup of the crumble, gently pressing it into the frosting. Top with the second cake layer. Cover the top and sides of the cake with a thin layer of frosting (this is the crumb coat—it doesn’t have to be perfect). Refrigerate until the frosting is set, 15 to 20 minutes. Cover with the remaining frosting. Press some of the remaining crumble around the bottom of the cake. Spoon the ganache over the top of the cake and gently spread, letting it drip down the sides. Sprinkle with the remaining crumble.