For the caramel filling

1. Whisk together sugar, 1/4 cup water, and corn syrup in a deep medium saucepan over low heat until sugar dissolves. Increase heat to medium; cover pan and cook 4 minutes. Uncover; increase heat to high. Boil without stirring until syrup is golden brown to deep amber in color, about 6 minutes. Remove from heat. Slowly add cream (mixture will bubble but will settle as you whisk it). Whisk in butter, then sour cream, lemon juice, and pinch of fleur de sel. Cool completely.
2. The caramel can be made 3 days ahead. Cover and chill. Bring to room temperature before using.

For the ganache filling and frosting

1. Place chocolate in large bowl. Bring cream to simmer in medium saucepan. Pour hot cream over chocolate; let soften 1 minute. Whisk until chocolate is smooth. Cool, then cover and chill overnight.
2. The ganache can be made 3 days ahead. Keep chilled. Bring to room temperature before using.

For the cake

1. Preheat oven to 350°F. Butter two 9-inch-diameter cake pans with 2-inch-high sides. Line bottom of each with parchment paper; butter paper and dust pan with flour. Set aside.
2. Combine sugar, flour, cocoa powder, baking powder, baking soda, and salt in a large bowl. Add milk, eggs, and melted butter. Using electric mixer, beat at low speed until blended. Increase speed and beat 2 minutes more. Stir 1 cup hot water and espresso powder in a small bowl to dissolve. Add to batter; beat until blended (batter will be thin). Divide batter between pans (about 3 – 3 1/2 cups each).
3. Bake cakes until tester inserted into center comes out clean, about 30-35 minutes. Cool cakes in pans on racks for 10 minutes. Cut around cakes; turn out onto racks. Peel off parchment and cool completely.

To assemble the cake

1. Using a long serrated knife, cut each cake horizontally in half. Place 1 layer on platter; spread with 1/2 cup room-temperature ganache. Spoon 3/4 cup ganache into a pastry bag fitted with 1/4-inch plain round tip (I used a disposable pastry bag with the tip cut off). Pipe ring of ganache around outside edge of cake layer. Spread 1/4 cup + 1 tablespoon room-temperature caramel filling evenly inside ring. Sprinkle caramel with large pinch of fleur de sel.
2. Top with second cake layer, repeat with ganache, ganache ring, caramel filling, and fleur de sel. Repeat again with third cake layer.
3. Top with fourth cake layer, cut side down. Spread remaining ganache over top and sides of cake.
4. Entire cake can be made 2 days ahead. Cover with cake dome and chill. Let cake stand at room temperature 1 hour before serving.