Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 13x18-inch (half sheet) baking pan, or line with parchment paper.
Step 2: In a large saucepan, combine butter, water, and cocoa powder. Bring to a boil over medium heat, stirring constantly until butter is melted and mixture is smooth. Remove from heat.
Step 3: In a large bowl, whisk together self-rising flour and granulated sugar.
Step 4: Pour the hot cocoa mixture into the dry ingredients and stir until well combined. The mixture will be thick.
Step 5: In a separate small bowl, whisk together buttermilk, eggs, vanilla extract, and baking soda until well combined.
Step 6: Add the buttermilk mixture to the chocolate mixture and stir until just combined and smooth. Do not overmix.
Step 7: Pour the batter into the prepared baking pan and spread evenly. Bake for 20-25 minutes, or until a toothpick inserted into the center comes out with moist crumbs.
Step 8: While the cake is baking, prepare the fudge frosting. In a medium saucepan, melt butter over medium heat. Add cocoa powder and milk, bringing to a boil, stirring constantly. Remove from heat.
Step 9: Gradually whisk in the powdered sugar until smooth. Stir in the vanilla extract.
Step 10: As soon as the cake comes out of the oven, immediately pour the warm fudge frosting evenly over the entire hot cake. Spread gently if needed.
Step 11: Drizzle the caramel sauce generously over the warm frosting. Sprinkle the chopped pecans evenly over the top. The warmth of the cake will help the frosting and caramel set slightly.
Step 12: Allow the cake to cool completely before slicing and serving. This allows the frosting and caramel to set properly for clean slices.