In a large bowl, mix flour, sugar, and baking soda until combined
In a saucepan, heat butter, cocoa powder, and milk until smooth but not boiling
Pour the warm chocolate mixture into the dry ingredients and mix well
Add eggs one at a time, mixing after each addition
Stir in vanilla extract until the batter is smooth
Grease a sheet pan and pour the batter evenly
Bake in a preheated oven at 180 C until the center is set, about 20 to 25 minutes
While the cake bakes, prepare the frosting by heating butter, cocoa powder, and milk in a saucepan
Remove from heat and stir in powdered sugar until smooth
Pour the warm frosting over the hot cake and spread evenly
Let the cake cool slightly before cutting and serving