Step 1: Preheat your oven to 350°F (175°C). Grease and flour a 13x18-inch sheet pan (or a 10x15-inch jelly roll pan for a slightly thicker cake).
Step 2: In a large bowl, whisk together the flour, granulated sugar, 1/4 cup cocoa powder, baking soda, and salt. Set aside.
Step 3: In a medium saucepan, combine 1 cup butter and 1 cup water. Bring to a rolling boil over medium heat, stirring occasionally. Remove from heat.
Step 4: Pour the hot butter mixture over the dry ingredients and stir until just combined. Don't overmix.
Step 5: Stir in the buttermilk, beaten eggs, and 1 teaspoon vanilla extract until the batter is smooth. The batter will be thin.
Step 6: Pour the batter evenly into the prepared sheet pan.
Step 7: Bake for 20-25 minutes, or until a wooden skewer inserted into the center comes out with moist crumbs. Do not overbake. While the cake is baking, prepare the frosting.
Step 8: For the frosting: In a medium saucepan, combine 1/2 cup butter, 1/4 cup cocoa powder, and 1/2 cup milk. Bring to a boil over medium heat, stirring constantly. Remove from heat.
Step 9: Gradually whisk in the powdered sugar until smooth. Stir in 1 teaspoon vanilla extract and the chopped pecans.
Step 10: While the cake is still warm (immediately after removing it from the oven), pour the warm frosting evenly over the entire surface of the cake. Spread gently with an offset spatula if needed. Allow the cake to cool completely, letting the frosting set before slicing. Store any leftovers in airtight Basics Glass Food Storage containers at room temperature for up to 3 days, or refrigerate for longer freshness.