1. Preheat the oven to 350ºF/180°C.
2. Grease a 13x18-inch pan. I serve it directly from the pan or cake tin, but if you want to remove it and serve it on a platter, you can line the sheet pan with a piece of parchment paper. Take a strip wide as the long side of the pan and place it covering the bottom and the two long sides. This way, it'll be easier to remove the cake by lifting the paper on both sides. 

For the cake:

1. Melt 1 cup of butter in a small saucepan.
2. Add cocoa and water and bring to a boil. Remove from heat.
3. Sift flour, baking soda, cinnamon, and salt in a large mixing bowl. Add the brown sugar and combine. 
4. Make a well in the center and add the cocoa mixture, 1/3 can (eyeball it) of the condensed milk, the eggs, and vanilla. Mix until the batter is smooth.
5. Pour into the prepared pan and bake for about 20 minutes, until the cake springs back and a cake tester or toothpick inserted in the center comes out with some moist crumbs attached. It should not come out wet. Don't overbake it. This is not a very tall cake so it doesn't take long to bake. But all ovens are different so check to see if it's done. 

For the frosting:

Have it ready before the cake comes out of the oven.

1. Melt the remaining ¼ cup of butter in a small saucepan (I use the same one as before without cleaning it).
2. Remove from heat and add the 2 tablespoons of cocoa powder, and the rest of the condensed milk. Mix well.
3. Add the powdered sugar and mix until completely smooth.
4. Spread over the warm chocolate cake as it comes out of the oven. You can poke holes with a toothpick or brochette stick so it seeps deeper. 
5. Let cool completely.
6. If removing it from the pan, refrigerate it first until firm so it's easier to remove. 
7. Refrigerate leftovers, well covered in plastic wrap or in an airtight container.