For the cake:
Unsalted butter, for pan
1 1/2 cups all-purpose flour, plus more for pan
3/4 cup unsweetened Dutch-process cocoa powder
1 1/2 cups granulated sugar
1 1/2 teaspoons baking soda
3/4 teaspoon baking powder
Salt
2 large eggs
3/4 cup warm water
3/4 cup low-fat buttermilk
3 tablespoons vegetable oil
1 teaspoon pure vanilla extract
For the Milk-Chocolate Crunch:
1 cup crisped-rice cereal
6 ounces milk chocolate, melted
For the caramel sauce:
1 cup granulated sugar
1 cup heavy cream
1/2 cup light corn syrup
3 tablespoons unsalted butter
1/4 teaspoon pure vanilla extract
Salt
For the frosting:
1/4 cup plus 2 tablespoons unsweetened Dutch-process cocoa powder
1/4 cup plus 2 tablespoons hot water
2 sticks unsalted butter
1/2 cup confectioners' sugar
Salt
1 pound semisweet chocolate, melted and cooled