Preheat an oven to 350* and spray an 8x8 pan with non-stick spray.
Start by mixing together the flour, baking soda, salt, white sugar, vinegar, milk, melted shortening, two tablespoons of cocoa powder, and nuts, if desired.
Once well mixed, pour into a baking pan. The batter will be quite thick.
Now, mix brown sugar, four tablespoons of cocoa powder, and boiling water together then pour this syrup over the batter mixture.
Bake 350* for 35 minutes. When done, it will be slightly crispy on top and gooey on the bottom, just like hot fudge. Delicious!