Preheat oven to 350 degrees. Make the cake: Butter and flour a 7-inch round cake pan (3 inches deep). Sift flour, cocoa powder, granulated sugar, baking soda, baking powder, and 3/4 teaspoon salt into a large bowl. Add eggs, warm water, buttermilk, oil, and vanilla. Beat with a mixer on medium-high speed until smooth, about 3 minutes.
Pour batter into prepared pan, and bake until a toothpick inserted into the center comes out clean, 55 to 60 minutes. Let cool completely in pan.
Meanwhile, make the milk-chocolate crunch: Fold crisped-rice cereal into melted milk chocolate until completely coated. Spread mixture about 1/4 inch thick onto a baking sheet lined with a nonstick baking mat. Freeze until set, about 45 minutes. Chop into small pieces. Refrigerate until ready to use.
Make the caramel sauce: Bring granulated sugar, cream, corn syrup, and butter to a boil in a medium saucepan over medium-high heat. Cook until mixture registers 236 degrees on a candy thermometer. Immediately remove from heat, and stir in vanilla and a pinch of salt. Transfer to a bowl, and let cool.
Make the frosting: Place cocoa powder in a small bowl, and stir in hot water to create a thick paste. Beat butter, confectioners' sugar, and a pinch of salt until pale and fluffy. Gradually beat in melted semisweet chocolate and cocoa paste. If frosting doesn't seem firm enough, refrigerate for 10 to 12 minutes, whisking every few minutes. Let stand at room temperature until ready to use.
Assemble the cake: Turn out cake from pan, and trim top. Slice into 3 even rounds. Place bottom layer on a rotating cake stand or a large plate, and spread 1/2 cup milk-chocolate crunch on top. Gently spread 3/4 cup caramel sauce on top, and cover with another 1/2 cup crunch. Top with middle cake layer, and repeat, finishing with top cake layer.
Frost cake with a crumb coat (a thin layer of frosting to seal in stray crumbs): Gently sweep away any loose crumbs with a pastry brush, then use a small offset spatula to cover the top and sides a with a thin layer of frosting. Refrigerate until set, about 1 hour.
Let crumb-coated cake stand at room temperature for 45 minutes. Frost entire cake with remaining frosting in a swirling motion using a large offset spatula. Let stand at room temperature until ready to serve (up to 8 hours).
Frosting can be stored at room temperature for up to 2 days.
