In a bowl combine all purpose flour, cocoa powder, baking powder, baking soda and castor sugar. Mix well.
Add milk and oil. Mix to make a batter.
Add vinegar and mix.
Divide the batter into 2 mugs and microwave at regular heating mode for 60 seconds.
Set the baked mug cakes aside.
In a pan, take milk and add cocoa powder and sugar. Cook on low flame while stirring until it warms up.
Take 2-3 spoons milk from the pan and mix it with corn flour to make a slurry.
Add the slurry back to the pan and bring the entire mixture to a boil while stirring continuously.
Once it comes to a boil, turn the flame to low and add chopped chocolate.
Stir to dissolve the chocolate.
Top the mug cakes with the thick hot chocolate and enjoy.
