Heat the oven and grease the sheet pan so the cake does not stick. The pan should look lightly coated but not greasy.
Mix the dry cake ingredients in a large bowl until the color looks even. There should be no white streaks left.
Heat butter, milk, and cocoa in a saucepan until melted and smooth. The mixture should look glossy, not grainy.
Pour the warm liquid into the bowl and stir until a smooth batter forms. The batter should be thin and pour easily.
Add the eggs and mix just until blended. The batter should look uniform with no streaks.
Pour the batter into the pan and spread it flat. Bake until the surface springs back when lightly touched.
While the cake bakes, melt butter with cocoa and milk for the icing. Stir in powdered sugar until smooth and thick.
Pour the warm icing over the hot cake and spread it gently. It should flow easily and cover the surface.
Sprinkle chopped nuts on top if using. Let the cake cool until the icing sets.