Preheat the oven to 350°F. Grease and flour two 9-inch round cake pans.
In a large bowl, whisk together sugar, flour, cocoa powder, baking soda, baking powder, and salt.
Add eggs, buttermilk, oil, and vanilla extract. Beat on medium speed for 2 minutes. Stir in boiling water (batter will be thin).
Divide batter between prepared pans and bake for 30–35 minutes. Let cool completely.
To make the ganache: heat 1 cup heavy cream in a saucepan until just simmering. Pour over chopped chocolate and stir until smooth. Let cool slightly.
For the caramel: melt butter in a pan over medium heat. Add brown sugar, 1/2 cup cream, and salt. Cook for 5 minutes, stirring. Remove from heat and stir in bourbon and toasted pecans.
To assemble: place one cake layer on a plate. Spread with half the ganache, then layer with caramel pecans. Top with second cake layer and cover with remaining ganache.
Chill until set. Slice and serve.