Preheat oven to 350 F.
Prepare the Pan - Spray or brush a 9 inch pan with oil. Line the base of the cake with parchment paper. Flour the sides of the cake pan with cocoa powder.
In a large bowl, sift the flour, cocoa powder, baking soda, baking powder and salt. Mix in the granulated sugar.
In another bowl, nicely whisk together milk, oil, egg and vanilla.
Make a well in the centre of the flour mixture and pour the wet ingredients. Fold in gently till just incorporated. Pour over the hot coffee and gently (dont not whisk) mox everything until you see a smooth shiny brown batter with no streaks of white flour. The batter will be runny.
Pour into the prepared pan and bake for 25-30 minutes or untill a skewer inserted in the centre of the cake comes out clean. The cake will start to pull away from the sides slightly.
Take out of the oven and let cool for 10 minutes in the pan. Run a butter knife all around and take out the cake out of the pan on to a cooling rack. Cool completely.
Once cooled, you can store the cake in the fridge for 2-3 days. This cake freezes easily, just wrap the cake in a cling film followed by placing it in a large ziploc and freeze upto a month.
For trifle, cut up the cake into small cubes using a sharp knife.