Step 1: Preheat your oven to 350°F (175°C) and grease a 10×15-inch or 12×17-inch jelly roll pan.
Step 2: In a large mixing bowl, whisk together the flour, sugar, baking soda, and salt. Set aside.
Step 3: In a medium saucepan, combine 1 cup of butter, water, and 1/4 cup of cocoa powder. Bring to a boil over medium heat, stirring constantly.
Step 4: Pour the boiling cocoa mixture over the dry ingredients and whisk until well combined.
Step 5: Stir in the buttermilk, eggs, and 1 teaspoon of vanilla extract until the batter is smooth.
Step 6: Pour the batter into the prepared pan and bake for 18-22 minutes, or until a toothpick inserted into the center comes out clean.
Step 7: About 5 minutes before the cake is done, prepare the frosting. In a medium saucepan, combine 1/2 cup butter, 1/4 cup cocoa powder, and milk. Bring to a boil.
Step 8: Remove from heat and whisk in the powdered sugar and vanilla extract until smooth.
Step 9: Immediately pour the warm frosting over the hot cake, spreading it evenly to the edges.
Step 10: Sprinkle the toasted pecans over the frosting while it is still wet.
Step 11: Drizzle the caramel sauce over the top of the pecans in a decorative pattern.
Step 12: Allow the cake to cool completely in the pan before slicing into squares.