Chocolate Cake with Caramel-Milk Chocolate Frosting

INGREDIENTS

• 1 cup unsweetened cocoa powder (preferably Dutch-process)
• 1 1/2 cups sugar
• 2 1/4 cups whipping cream
• 2 cups sifted cake flour (sifted, then measured)
• 1 cup plus 2 tablespoons (2 1/2 sticks) unsalted butter, room temperature
• 1/2 teaspoon baking powder
• 24 ounces imported milk chocolate (such as Lindt), finely chopped
• 3 ounces bittersweet (not unsweetened) or semisweet chocolate, finely chopped
• 1 cup plus 2 tablespoons buttermilk
• 1 tablespoon vanilla extract
• 1 1/4 teaspoons baking soda
• 3/4 teaspoon salt
• 1/2 cup lukewarm water
• 1 1/2 cups (packed) golden brown sugar
• 1 cup semisweet chocolate chips
• 3 large eggs
• Nonstick vegetable oil spray
• 1/2 cup water