To prepare cake: Preheat oven to 350 degrees. Butter bottom and sides of 3 9-inch cake pans and line bottom of each pan with parchment paper. Butter the paper; dust pans with unsweetened cocoa powder and set pans aside.
In a small bowl, stir together egg, buttermilk and oi and reserve. In a large bowl, stir together flour, sugar, 1 cup cocoa powder, baking soda and salt. Using an electric mixer on low speed, gradually add buttermilk mixture to flour mixture, beating until combined. Gradually beat in coffee.
Divide batter evenly between prepared pans; layers will appear shallow. (If you only have two 9-inch cake pans, refrigerate 1 of the batter while you bake the other two layers. Clean and prepare one of the pans as directed, and bake remaining batter.)
Bake until a wooden toothpick inserted near center comes out clean, about 22 to 25 minutes. Remove from oven and cool cakes on a wire racks for 10 minutes. Loosen sides from pans, then invert cakes on racks. Remove cakes from pans. Peel off paper; cool completely.nnTo prepare frosting: In a medium saucepan, combine sugar and milk. Add butter. Bring to a boil, stirring constantly. Remove from heat. Add semisweet chocolate pieces. Using a wire whisk, stir until smooth. If frosting is too thick or grainy, stir in 1 to 2 teaspoons freshly brewed coffee. If necessary, let frosting stand several minutes before using.
To assemble cake: When the cakes are cool, place 1 cake layer, top-side down, onto a serving plate with a lip. Using an icing spatula or wide knife, work quickly to frost the top of this layer with one-third of the frosting. Arrange one-third of the pecans on top and drizzle with some of the caramel topping. Top with the second layer, top-side down.
Repeat with frosting, pecans and caramel topping. Top with the third layer, top side up. Repeat with remaining frosting, pecans and caramel topping. Chill cake 1 to 2 hours before serving (some caramel topping will ooze from the cake as it chills).