Step 1: In a large saucepan, combine the water, butter, and cocoa powder. Bring to a boil over medium heat, stirring constantly until the butter is melted and the cocoa is fully dissolved. Remove from heat and let cool slightly for about 5 minutes.
Step 2: In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking soda, and salt. Make sure there are no lumps.
Step 3: Pour the warm chocolate mixture from the saucepan into the dry ingredients. Mix with an electric mixer on low speed until just combined. Don't overmix!
Step 4: In a separate small bowl, whisk together the buttermilk, eggs, and vanilla extract until well combined.
Step 5: Gradually add the buttermilk mixture to the chocolate batter, mixing on low speed until smooth. The batter will be thin, which is exactly what we want.
Step 6: Pour the thin batter evenly into your prepared 13x18-inch baking sheet.
Step 7: Bake for 20-25 minutes, or until a wooden skewer inserted into the center comes out clean. Keep a close eye on it to prevent overbaking.
Step 8: While the cake is still warm, start preparing the frosting. This is crucial for the frosting to melt slightly into the cake, creating that signature fudgy texture.
Step 9: In a medium saucepan, combine the butter, milk, and cocoa powder. Bring to a boil over medium heat, stirring constantly, until the butter is melted and the cocoa is dissolved.
Step 10: Remove from heat. Immediately add the powdered sugar and vanilla extract. Whisk vigorously until smooth.
Step 11: Stir in the chopped pecans (if using). If the frosting seems too thick, add a tiny bit more milk, one teaspoon at a time, until it reaches a pourable, but still thick, consistency.
Step 12: Pour the warm frosting evenly over the warm cake. Using an offset spatula, quickly spread it to the edges. Let the cake cool completely on a wire rack before cutting and serving.