Preheat the oven at 350 degree F or 180 degree C.
Prepare 9 inch round cake pan. Spray it with cooking spray or lightly butter the pan.
Sift all purpose flour, cocoa powder, baking powder and baking soda in a bowl. Then add water, melted butter, vanilla extract and condensed milk. Mix well using whisk. If there are lumps don’t worry about it.
Pour the batter into a prepared cake pan. Tap the pan on the countertop to remove air bubbles. And bake into preheated oven for 30 minutes or till toothpick inserted in center of cake will come out clean.
Remove it from the oven and let it cool on wire rack for 5 minutes. Then using a knife loose the sides of the pan. And invert it into cooling rack from the pan and let it cool completely before frosting.
Chop semi sweet chocolates and put into a heatproof bowl.
Heat the heavy cream and butter in a sauce pan on medium heat. As soon as it comes to a boil. Turn off the stove and pour hot cream and butter mixture into chocolate.
Let it stand for 2 minutes. After that it will look like as shown in first picture below. Then stir very gently with a spoon till it becomes shiny and everything is incorporated well.
Put the cake with cooling rack on a baking dish as shown below. So any access drip from frosting will not be wasted. Now pour the chocolate ganache frosting on the cake and using back of spoon or butter knife. Spread it evenly.
Let the ganache frosting set for 1 2 hours before cut the cake.
Do not over mix the batter.
Check the cake using toothpick before five minutes of mentioned time. And bake accordingly because everybody oven temperature may vary a little bit.
Let cake cool completely before frosting.
It stays fresh up to a week at room temperature.