Combine cream and corn syrup in a saucepan over medium heat until it simmers.Pour the hot mixture over chocolate in a microwave-safe bowl, add salt, and let it sit for 2 minutes.
Whisk until smooth and cool to room temperature (approximately 1 hour).Chill in the refrigerator until thickened and spreadable (about 45 minutes), stirring halfway through.
Preheat the oven to 350°F (175°C). Grease nine 8 1/2-inch round disposable cake pans and line them with parchment paper.Beat butter until creamy, add sugar gradually, and beat until light and fluffy (about 3 minutes).
Add eggs one at a time, beating after each addition. Beat in vanilla.Whisk together flour, baking powder, baking soda, and salt. Add this mixture to the butter mixture alternately with buttermilk in 3 batches.
Divide batter evenly among the pans and smooth with an offset spatula.Bake in batches until a toothpick comes out clean (12 to 14 minutes).Cool in pans for 10 minutes, then transfer cakes to wire racks to cool completely (about 20 minutes).
Place 1 cake layer on an 8-inch round cake board and spread with 1/3 cup icing. Top with another layer and repeat with remaining layers and icing. Coat the top and sides of the cake with a thin layer of icing. Chill the cake in the refrigerator for 1 hour.
Place the chilled cake on a wire rack over a baking pan.Microwave reserved icing in intervals until smooth and pourable (60 to 90 seconds).Pour icing over the cake in a steady stream, starting at the center and moving to the edges.Smooth out the sides and fill any gaps with an offset spatula.
Chill the cake in the refrigerator until set (about 1 hour).Carefully transfer the cake to a serving plate.Serve the cake at room temperature. Enjoy your delicious homemade treat!