firstly, in a large mixing bowl take ½ cup butter and 1 cup sugar.
beat smooth and fluffy till the butter and sugar combine well.
now add 1 cup milk, 1 tsp vinegar and 1 tsp vanilla extract. mix well.
sieve 1½ cup maida, ½ cup custard powder, ½ tsp baking soda and 1 tsp baking powder.
mix well using cut and fold method.
transfer the cake batter into the round cake mould and bake at 180 degree celsius for 40 minutes.
now spread a generous amount of jam mixture over cake and level out forming a frosting.
finally, serve custard cake immediately, or store in a refrigerator for a week.