Step 1: Preheat your oven to 375°F (190°C). Grease and lightly flour a 13x18 inch jelly roll pan (or a standard half-sheet pan).
Step 2: In a large, heavy-bottomed saucepan, combine 1 cup unsalted butter, 1 cup water, and 1/4 cup unsweetened cocoa powder. Bring to a rolling boil over medium heat, stirring constantly. Once boiling, remove from heat.
Step 3: In a very large mixing bowl, whisk together 2 cups all-purpose flour, 2 cups granulated sugar, 1/2 teaspoon baking soda, and 1/2 teaspoon salt.
Step 4: Pour the hot butter-cocoa mixture into the dry ingredients and mix well with a whisk until just combined. Do not overmix.
Step 5: In a separate small bowl, whisk together 2 large eggs, 1/2 cup buttermilk, and 1 teaspoon vanilla extract. Add this to the chocolate mixture and mix until smooth.
Step 6: Pour the batter evenly into the prepared jelly roll pan. Bake for 18-22 minutes, or until a wooden skewer inserted into the center comes out clean.
Step 7: While the cake is baking, prepare the frosting. In a medium saucepan, combine 1/2 cup unsalted butter, 1/4 cup buttermilk, 1/4 cup milk, and 1/4 cup unsweetened cocoa powder. Bring to a boil over medium heat, stirring frequently.
Step 8: Remove from heat and immediately stir in 4 cups powdered sugar, 1 teaspoon vanilla extract, and a pinch of salt until smooth and free of lumps. Work quickly.
Step 9: While the frosting is still warm, pour it evenly over the warm cake immediately after it comes out of the oven. Use an offset spatula to spread if needed.
Step 10: Immediately after frosting, sprinkle 1 cup of your chopped, toasted pecans evenly over the entire warm, frosted cake. Gently press them down slightly so they adhere.
Step 11: Drizzle 1/2 cup of caramel sauce generously over the entire cake.
Step 12: Allow the cake to cool completely, preferably at room temperature, before cutting into squares and serving. This allows the frosting to set and flavors to meld.