Step 1: Preheat your oven to 350°F (175°C). Lightly grease and flour a 15x10-inch jelly roll pan.
Step 2: In a large mixing bowl, whisk together 2 cups all-purpose flour, 2 cups granulated sugar, 2 tablespoons cocoa powder, 1 teaspoon baking soda, and ½ teaspoon salt until well combined.
Step 3: In a separate medium bowl, lightly beat 2 room-temperature eggs. Stir in 1 cup buttermilk, ½ cup vegetable oil, and 1 teaspoon vanilla extract until smooth.
Step 4: Pour the wet mixture into the dry ingredients. Mix until just moistened; be careful not to overmix, as this can lead to a tough cake.
Step 5: Slowly add 1 cup hot water to the batter, stirring until the batter is thin and free of lumps.
Step 6: Pour the thin batter evenly into your prepared jelly roll pan.
Step 7: Bake for 20-25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Do not overbake.
Step 8: Remove the cake from the oven and place it on a wire rack to cool slightly while you prepare the frosting.
Step 9: For the frosting, in a small saucepan over medium heat, combine ½ cup unsalted butter, ¼ cup milk, and ¼ cup cocoa powder. Stir constantly until the mixture comes to a gentle boil around the edges.
Step 10: Remove the saucepan from the heat. Whisk in 3 cups powdered sugar and 1 teaspoon vanilla extract until the frosting is completely smooth.
Step 11: If using, fold in ½ cup chopped pecans for added texture and flavor.
Step 12: Immediately pour the warm frosting evenly over the warm cake, spreading quickly to the edges. The warmth of the cake and frosting will allow the icing to seep beautifully into the cake.