This cake is based on one from Cook's Illustrated. There are some options here if you want to simplify: while the recipe calls for slicing each 9-inch cake into two rounds for four total layers, the cake is equally wonderful as two layers, without slicing it (use just 1/3 of the caramel to fill between the two layers). The caramel filling is special, yet I often turn to a good jar of dulce de leche or other thick caramel sauce. Or, replace the filling with more of the meringue frosting instead.
The cake can be made a day in advance; if you’re making the caramel filling, fill the cake layers, then build and refrigerate the cake, covered with plastic wrap. If you’re not using the caramel filling, just wrap the layers uncut in plastic wrap and refrigerate them for up to one day. Make the frosting and frost the cake the same day it is going to be served. Frosting a cold cake straight from the refrigerator is much easier than frosting a room temperature cake (fewer crumbs).
Adjust an oven rack to the middle position and heat the 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 a large bowl. Whisk in sugar, baking soda, baking powder, and salt. Whisk buttermilk, water, oil, eggs, vanilla and espresso powder together in a second bowl or large measuring cup. Whisk buttermilk mixture into flour mixture until a smooth batter forms. Divide batter evenly between prepared pans and smooth tops with rubber spatula.
Bake the cake until a toothpick inserted in center comes out clean, 22 to 28 minutes, switching and rotating pans halfway through baking. Let the cakes cool in pans on wire rack for 15 minutes. Remove cakes from pans, discarding parchment, and let cool completely on rack.
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.)
Off heat, carefully stir in cream, butter, vanilla, orange blossom water, 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.
In the bowl of a stand mixer, stir together the egg whites, sugar, cream of tartar, and salt. I use the whisk attachment to stir. Set the bowl over a saucepan of simmering water, making sure that the bowl has some clearance above the water (we're cooking very gently with the residual or steam heat here). Heat, stirring frequently, until the mixture reaches 160 degrees, about 8 minutes.
Transfer the whites to the stand mixer with the wire whisk attached. Beat, starting slow and increasing the speed steadily, until the mixer is on full. Whip until the stiff, glossy peaks form, 5 to 7 minutes. Stir in the vanilla and beat briefly.
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.
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.)
Line edges of a cake stand or platter with 4 strips of parchment to keep the cake 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. Now you can cover and refrigerate the cake for up to one day.
Spread the marshmallow frosting thickly and evenly over sides and top of cake, making peaks and valleys with an offset spatula or the back of a spoon. Take care not to get chocolate crumbs in the beautiful white meringue.
Toast the marshmallow by lightly skimming the surface with a culinary torch.
Carefully remove parchment strips. Leave the cake out at room temperature until serving, same (special) day.