01 - Preheat oven to 350°F. Grease two 9-inch round cake pans and line with parchment paper.
02 - Whisk together flour, sugar, cocoa powder, baking powder, baking soda, and salt in a large bowl.
03 - Add buttermilk, vegetable oil, eggs, and vanilla. Beat on medium speed until well combined.
04 - Gradually mix in hot water until batter is smooth. The batter will be thin.
05 - Divide batter evenly between prepared pans. Bake for 32-36 minutes until toothpick inserted in center comes out clean.
06 - Let cakes cool in pans for 10 minutes, then turn out onto wire racks to cool completely.
07 - Place chopped chocolate in heatproof bowl. Heat cream in small saucepan until just simmering. Pour over chocolate and let sit 2 minutes. Add butter and stir until smooth and glossy. Cool until thickened but pourable.
08 - Place one cake layer on serving plate. Spread with half the caramel sauce and sprinkle with half the chopped pecans. Drizzle with some ganache.
09 - Top with second cake layer. Pour ganache over top, letting it drip down sides. Spread evenly with spatula if needed.
10 - Decorate with remaining caramel sauce, pecan halves, and additional ganache. Chill for 30 minutes before slicing for cleaner layers.