Step 1: Preheat oven to 375°F (190°C). Grease and flour a 13x18 inch baking sheet (half-sheet pan).
Step 2: In a large saucepan, melt 1 cup butter. Add 1 cup water and 1/2 cup cocoa powder. Bring mixture to a boil over medium heat, stirring constantly. Remove from heat and let cool for 5 minutes.
Step 3: In a large bowl, whisk together flour, granulated sugar, baking soda, and salt. Pour the cooled chocolate mixture into the dry ingredients and mix well.
Step 4: In a separate bowl, whisk the buttermilk, eggs, and 1 teaspoon vanilla extract. Add this to the chocolate mixture and stir until just combined. The batter will be thin.
Step 5: Pour the batter evenly onto the prepared baking sheet. Bake for 18-22 minutes, or until a wooden skewer inserted into the center comes out with moist crumbs. Do not overbake. While the cake bakes, prepare the caramel layer.
Step 6: For the Caramel Layer: In a heavy-bottomed saucepan, combine 1/2 cup butter, brown sugar, and 1/4 cup heavy cream. Bring to a boil over medium heat, stirring constantly. Reduce heat and simmer for 5-7 minutes, continuing to stir, until thickened. Remove from heat.
Step 7: Once the cake is out of the oven, immediately pour the warm caramel evenly over the hot cake. Spread it gently with an offset spatula. Sprinkle 1 cup of the toasted pecans over the caramel layer. Allow to cool slightly (about 10 minutes) while you prepare the fudge frosting.
Step 8: For the Fudge Frosting: In a medium saucepan, melt 1/2 cup butter. Add 1/4 cup cocoa powder and 1/4 cup milk, bringing to a boil while stirring constantly. Remove from heat.
Step 9: Whisk in the powdered sugar and 1 teaspoon vanilla extract until smooth. The frosting should be warm and pourable. If too thick, add a tiny bit more milk; if too thin, a bit more powdered sugar.
Step 10: While the cake is still warm (but not piping hot), pour the warm fudge frosting evenly over the caramel and pecans. Use an offset spatula to spread quickly if needed.
Step 11: Immediately sprinkle the remaining 1/2 cup of toasted pecans over the wet frosting. Allow the cake to cool completely at room temperature for at least 2-3 hours before slicing and serving. This allows the frosting and caramel to set properly.