Preheat the oven to 175°C (350°F) and grease a large sheet cake pan.
Combine flour, sugar, cocoa powder, baking soda, and salt in a mixing bowl.
Heat butter, water, and cocoa powder until melted and smooth, then mix with the dry ingredients.
Add eggs, buttermilk, and vanilla extract to create a smooth cake batter.
Pour the batter into the sheet pan and bake for 20–25 minutes until set.
Prepare the chocolate frosting by heating butter, cocoa, milk, and powdered sugar.
Spread warm frosting over the cake, add chopped pecans, and allow it to cool before serving.