Preheat the oven to 350°F (175°C). Grease your cake pan and set it aside. In a large mixing bowl, combine the flour, sugar, baking soda, salt and cocoa powder. Give everything a good whisk so the ingredients are evenly mixed.

Add the butter and boiling water to a medium saucepan over medium heat. Stir gently until the butter has completely melted and the mixture is hot. Remove the saucepan from the heat, then slowly pour the warm mixture into the bowl of dry ingredients, stirring as you pour. Continue stirring until the mixture is well combined and smooth.

Pour in the buttermilk, beaten eggs, and vanilla extract. Stir gently until you have a smooth batter with no visible pockets of flour. Try not to overmix, as too much stirring can make the finished cake dense instead of soft and tender.

Pour the batter into the greased pan and smooth the surface with a spatula. Place it in the preheated oven and bake for 25–30 minutes. The cake is ready when a toothpick inserted into the center comes out with a few moist crumbs attached. Take care not to overbake it. Let the cake cool slightly while you prepare the icing.

Melt the butter in a saucepan over medium-low heat. Add the cocoa powder and milk, whisking until the mixture is smooth and well blended. Remove the pan from the heat and stir in the vanilla extract. Gradually add the powdered sugar, whisking until the icing is smooth, creamy, and glossy. Stir in the chopped pecans or walnuts.

Spread the warm chocolate icing over the cake while it is still warm. Use a spatula to spread it evenly from edge to edge. Let the icing sit for a few minutes to firm up, then cut the cake into 12 pieces. Serve at room temperature or enjoy it slightly warm.