Salted Caramel Chocolate Cake

Adapted from Hershey’s, Sweet Paul, and
Makes a 2-layer cake (about 16 servings)
XIAOLU'S NOTES: I recommend making the filling before the cake layers so the first has time to cool; it can be made up to 3 days ahead. I actually reheated my filling just a tad too much, which is why it's runny. Keep a careful eye on it, and it should remain firm enough to stay put. The frosting can also be mixed up ahead of time, chilled, and brought to room temperature before using.
1 3/4 cups all purpose flour
2 cups sugar
3/4 cup unsweetened cocoa powder
2 tsp baking soda
1 tsp baking powder
1 tsp salt
2 large eggs, room temperature
1 cup strong black coffee OR 2 tsp instant coffee powder plus 1 cup boiling water
1 cup buttermilk OR 1 Tbsp apple cider vinegar plus milk to equal 1 cup
1/2 cup vegetable oil
2 tsp pure vanilla extract
Salted Caramel Filling (Recipe below)
Chocolate Buttercream Frosting (Recipe below)
Extra fleur de sel OR other fine-grained sea salt, to garnish
Preheat the oven to 350 degrees F. Grease and dust with cocoa powder two 9-inch round pans. Optional but helpful: line pans with parchment circles.
In large bowl, sift and then stir together the flour, sugar, cocoa, baking soda, baking powder, and salt. Add the eggs, buttermilk (or milk vinegar mixture), coffee (or reconstituted coffee mixture), oil, and vanilla extract. Beat by hand or with an electric mixer on medium speed until well combined.
Pour the batter, which will be thin, into the prepared pans. Tap the pans firmly against the counter one time to remove any air bubbles. Bake just until a toothpick inserted into the middle of the cakes comes out clean (about 35-40 minutes). Let cakes cool in the pans for 10 minutes, then remove to wire racks and let them cool completely there.
Place a small dollop of frosting in the center of a cake plate or stand and top with one cool cake layer. Warm the caramel filling still in the saucepan over low heat for 5 seconds at a time, stirring in between, until of thick spreadable consistency. Spread evenly over the cake. Place the second cake layer over the filling and frost the top with chocolate buttercream. Finish the cake off with a sprinkle of sea salt, if desired.
Serve immediately or refrigerate until needed. Cake will keep for 4 or more days if well covered.
Salted Caramel Filling
1/2 cup sugar
3 Tbsp water
1 Tbsp light corn syrup
1/3 cup heavy cream
2 Tbsp unsalted butter
3/4 tsp fleur de sel OR other fine-grained sea salt
Place the sugar, water, and corn syrup in a large heavy-bottomed saucepan over low heat. Stir until the sugar is dissolved. Turn the heat up to medium and allow the mixture to cook without stirring until the sugar turns a dark amber color (about 10 minutes). Remove the pan from heat and pour in the cream (the mixture will bubble up). Stir well, then stir in the butter and salt. Let the filling cool to room temperature. Then cover the pot and chill in the refrigerator until needed.
Chocolate Buttercream Frosting
1/2 cup (1 stick) unsalted butter, softened 
3/4 cup unsweetened cocoa powder 
2 cups powdered sugar, unsifted
Large pinch salt 
5 Tbsp milk, half and half, OR heavy cream 
1 tsp pure vanilla extract
Sift the cocoa powder, powdered sugar, and salt into a small bowl. Cream the butter with an electric mixer in another bowl. Alternate adding the sifted mixture with adding the cream, mixing well between each addition. Continue beating until the desired texture is achieved. Add more cream or powdered sugar if you want to thin or thicken the frosting.