Many chocolate-caramel cakes barely contain enough caramel flavor to merit the name. To ensure caramel flavor in each bite, we sandwich three layers of thick but spreadable caramel filling between layers of deep, dark, moist chocolate cake. For a not-too-sweet caramel that is spreadable but thick enough to stand out between the layers, we cook it until it’s dark (but not burnt) and add extra butter to ensure that it sets up at room temperature and does not ooze. Our thick, satiny chocolate frosting comes together in the food processor in minutes.

Cake

• 1 ½ cups (7 1/2 ounces) all-purpose flour
• ¾ cup (2 1/4 ounces) unsweetened cocoa powder
• 1 ½ cups (10 1/2 ounces) granulated sugar
• 1 ¼ teaspoons baking soda
• ¾ teaspoon baking powder
• ¾ teaspoon salt
• ¾ cup buttermilk
• ½ cup water
• ¼ cup vegetable oil
• 2 large eggs
• 1 teaspoon vanilla extract

Caramel Filling

• 1 ¼ cups (8 3/4 ounces) granulated sugar
• ¼ cup light corn syrup
• ¼ cup water
• 1 cup heavy cream
• 8 tablespoons unsalted butter, cut into 8 pieces
• 1 teaspoon vanilla extract
• ¾ teaspoon salt

Frosting

• 16 tablespoons unsalted butter, softened
• ¾ cup (3 ounces) confectioners' sugar
• ½ cup (1 1/2 ounces) unsweetened cocoa powder
• Pinch salt
• ½ cup light corn syrup
• ¾ teaspoon vanilla extract
• 6 ounces bittersweet chocolate, melted and cooled
• ¼—½ teaspoon coarse sea salt, (optional)

1. FOR THE CAKE: Adjust oven rack to middle position and heat oven to 325 degrees. Grease two 9-inch round cake pans, line with parchment paper, grease parchment, and flour pans. Sift flour and cocoa into large bowl. Whisk in sugar, baking soda, baking powder, and salt. Whisk buttermilk, water, oil, eggs, and vanilla together in second bowl. Whisk buttermilk mixture into flour mixture until smooth batter forms. Divide batter evenly between prepared pans and smooth tops with rubber spatula.
2. Bake until toothpick inserted in center comes out clean, 22 to 28 minutes, switching and rotating pans halfway through baking. Let cakes cool in pans on wire rack for 15 minutes. Remove cakes from pans, discarding parchment, and let cool completely on rack, at least 2 hours.
3. FOR THE CARAMEL FILLING: Lightly grease 8-inch square baking pan. Combine sugar, corn syrup, and water in medium saucepan. Bring to boil over medium-high heat and cook, without stirring, until mixture is amber colored, 8 to 10 minutes. Reduce heat to low and continue to cook, swirling saucepan occasionally, until dark amber, 2 to 5 minutes longer. (Caramel will register between 375 and 380 degrees.)
4. Off heat, carefully stir in cream, butter, vanilla, and salt (mixture will bubble and steam). Return saucepan to medium heat and cook, stirring frequently, until smooth and caramel reaches 240 to 245 degrees, 3 to 5 minutes. Carefully transfer caramel to prepared pan and let cool until just warm to touch (100 to 105 degrees), 20 to 30 minutes.
5. FOR THE FROSTING: Process butter, sugar, cocoa, and salt in food processor until smooth, about 30 seconds, scraping down sides of bowl as needed. Add corn syrup and vanilla and process until just combined, 5 to 10 seconds. Scrape down sides of bowl, then add chocolate and pulse until smooth and creamy, 10 to 15 seconds. (Frosting can be made 3 hours in advance. For longer storage, cover and refrigerate frosting. Let stand at room temperature for 1 hour before using.)
6. Using long serrated knife, score 1 horizontal line around sides of each cake layer; then, following scored lines, cut each layer into 2 even layers.
7. Using rubber spatula or large spoon, transfer 1/3 of caramel to center of 1 cake layer and use small offset spatula to spread over surface, leaving 1/2-inch border around edge. Repeat with remaining caramel and 2 of remaining cake layers. (Three of your cake layers should be topped with caramel.)
8. Line edges of cake platter with 4 strips of parchment to keep platter clean. Place 1 caramel-covered cake layer on platter. Top with second caramel-covered layer. Repeat with third caramel-covered layer and top with final layer. Spread frosting evenly over sides and top of cake. Carefully remove parchment strips. Let cake stand for at least 1 hour. (Cake can be made 2 days in advance and refrigerated. Let stand at room temperature for at least 5 hours before serving.) Sprinkle with coarse sea salt, if using. Cut and serve.

NOTE FROM THE TEST KITCHEN: Baking spray that contains flour can be used to grease and flour the pans. Both natural and Dutch-processed cocoa will work in this recipe. When taking the temperature of the caramel in steps 3 and 4, remove the pot from the heat and tilt the pan to one side. Use your thermometer to stir the caramel back and forth to equalize hot and cool spots and make sure you are getting an accurate reading.