Step 1: Preheat your oven to 375°F (190°C). Grease and flour a 13×18 inch (half-sheet) baking pan.
Step 2: In a large bowl, whisk together the flour, granulated sugar, cocoa powder, baking soda, and salt. Set aside.
Step 3: In a medium saucepan, combine the butter and water. Bring to a boil over medium heat, then immediately remove from heat.
Step 4: Pour the hot butter mixture over the dry ingredients. Mix with a spoon or spatula until just combined.
Step 5: In a separate small bowl, whisk together the buttermilk, eggs, and vanilla extract. Add this mixture to the chocolate batter and mix until smooth. Do not overmix.
Step 6: Pour the batter into the prepared baking pan and spread evenly.
Step 7: Bake for 18-22 minutes, or until a wooden skewer inserted into the center comes out clean.
Step 8: While the cake bakes, prepare the fudge frosting. In a medium saucepan, melt the butter over medium heat. Add the milk and cocoa powder, whisking until smooth. Bring to a gentle boil, then remove from heat.
Step 9: Gradually whisk in the sifted powdered sugar until smooth. Stir in the vanilla extract. The frosting should be warm and pourable.
Step 10: As soon as the cake comes out of the oven, pour the warm fudge frosting over the hot cake, spreading it evenly to the edges.
Step 11: Immediately sprinkle the toasted pecans over the warm frosting.
Step 12: Drizzle the caramel sauce generously over the pecans and frosting.
Step 13: Allow the cake to cool completely before slicing and serving. This allows the frosting and caramel to set.