Preheat the oven to 180c/356f.
Using a hand mixer, mix the sugar into the butter until it is light and fluffy.
Add the 3 eggs, the milk, the vanilla essence and mix again for a couple more minutes and until it is light and creamy.
Add the self raising flour and the custard powder and with a fork mix well so that everything is well mixed into your batter and that you have that light cake batter mixture.
Use a little extra butter and rub it into the bottom and sides of your mould. Then follow this up with dusting it with some flour so that it makes the mould completely work when it has been cooked.
Place the custard cream batter into the mould and fill it as far as it will go. Then place it in the centre of the oven and cook for 30 minutes. Or until a tooth pick comes out clean.
Allow it to cool and then place a butter knife around the edges to loosen it and gently remove it from the mould. Slice the mould into two separate sandwich parts.
Make the custard filling by placing all the ingredients into a mixing bowl and using a hand mixer mix until it is light and fluffy.
Using a butter knife paste the custard cream filling onto the bottom sandwich and then sandwich the other one on top.
Place in the fridge for 30 minutes to cool and then serve.