1. Sieve together flour, corn flour and baking powder together at least thrice. Powder the sugar and keep aside. Grease and flour an 8 inch pan and keep it ready.Beat butter till fluffy, add powdered sugar and beat well.
2. Add condensed milk and beat once more.
3. Add milk, mix well and switch to a spatula.
4. Add flour mixture in batches and mix in the same direction.Finally add vanilla essence, mix well and transfer the batter to the baking pan.Preheat the oven at 180 C (360 F) and bake for 40-45 minutes or till a tooth pick inserted comes out clean.
5. Let the cake cool down completely and invert it onto a plate to do frosting.For the Chocolate Buttercream Frosting:Make sure that the butter is in room temperature and softened. Sift together cocoa powder and powdered sugar and keep aside.In a mixing bowl cream butter, add the sifted dry ingredients into the bowl and mix till the sugar is absorbed by the butter well.Now add in vanilla essence, salt and milk and beat for 2-3 minutes. I used my hand mixer but you can also use the balloon whisk for the mixing.If you want the frosting to be thin, spreading consistency, add more milk by 1 tbsp at a time and in case if you want the frosting to be stiff consistency to pipe it out, add more sugar little at a time to get the exact preferred consistency.
6. Spread on the cake all over evenly and decorate either with colored sugar sprinkles, silver balls or chocolate curls.