Preheat the Oven: Preheat your oven to 350°F (175°C). Grease a baking dish to prevent sticking.
Prepare the Cake Batter: In a mixing bowl, combine flour, sugar, cocoa powder, baking powder, and salt. Add milk, melted butter, and vanilla extract. Mix until smooth. Optionally, fold in chopped walnuts for added texture.
Bake the Cake: Pour the batter into the greased baking dish. Place in the oven already heated to 350°F and bake for around 25-30 minutes, or until a toothpick poked into the center comes out clean.
Prepare the Hot Fudge Sauce: In a saucepan, whisk together sugar and cocoa powder for the hot fudge sauce. Gradually add boiling water, stirring continuously until the mixture thickens.
Serve Warm: Once the cake is baked, allow it to cool slightly. Cut into squares, drizzle generously with the hot fudge sauce, and serve warm.