Pre-heat oven to moderate heat (350 to 375 F, 180-190 C or Gas Mark 4).
Grease 2 eight inch (20 cm) round cake pans. Line bases with baking paper.
Using an electric mixer, cream butter, vanilla essence, and sugar until light and fluffy; beat in eggs one at a time until fully blended.
Stir in sifted flour and milk in two batches. Spread mixture into both pans and bake in the pre-heated oven for approximately 30 minutes. (Note: Time will vary depending upon the efficiency of your oven. Your sponge cake is cooked when you can push a skewer into the middle and it pulls out clean.)
Turn out sponge cakes onto wire racks to cool.
While sponges cook and cool, there is ample time to make the filling as follows:
Combine butter, sugar, brandy and the water in a small saucepan, stir over low heat until the butter is melted and sugar dissolved. Bring to boil then boil rapidly, uncovered, for 1 minute without any stirring.
Remove from heat and gradually whisk in the egg until mixture slightly thickens. Transfer mixture to a bowl and mix in the remaining ingredients. Cover and refrigerate for a couple of hours until mixture holds its shape.
When filling is ready, place one cake on serving plate and spread filling evenly over the sponge cake, then top with the second sponge. Cover and refrigerate for a couple of hours. The frosting will be added just before serving.