The traditional way to make a Victoria Sandwich is to use the weight of the eggs (in their shells) to measure the Butter, flour and sugar. This method was created a long time before the advent of digital scales and the grading of eggs. This would ensure that the ratio of the ingredients were correct every time . 
Large eggs can vary between 63g-73g so this method is still a good way to get consistent results.
Weigh 3 Large eggs (in shell) and make a note of the total , this will be the weight for your flour, butter and sugar

You will require 2 x 19-20cm (7-8in) round sandwich tins
using the tins as a guide cut out 2 discs of greaseproof/baking parchment.
Lightly grease the tins and place the discs in the bottom of the tins.

All ingredients should be at room temp for at least 30 mins before starting. 

Turn on oven... 180C, 160C fan, 360F, Gas 4

Using either the weight of the 3 eggs or the weights given:

6oz (175g) Butter.
6oz (175g) Caster Sugar (cane is best if possible).
3 large eggs

6oz (175g) Plain Flour
1level teaspoon Baking powder sieved together 3 times.
6oz (175g) Self Raising Flour (Sieved)

In a suitable mixing bowl place the butter and sugar.
Using either a large spoon or hand mixer cream together until light and soft.
Crack the eggs into a separate container and lightly whisk together (a fork will do)
Gradually add the eggs in 4 lots to the fat/sugar, beating after each addition.
Care must be taken to avoid curdling If this start to happen add small amounts of flour with each addition.
When all the egg has been added, give a final beat .
Add the flour and carefully fold in until it forms a clear and uniform mixing,
It is important not to over mix as this will toughen the end product,
But it is also important to ensure that the flour is fully dispersed.

Divide the mixture evenly between the 2 tins
Using the back of a large spoon spread the mix evenly in the pans.

Place in the preheated oven and bake for aprox 20 mins +  
This will depend on individual ovens.
The product should be firm to a gentle touch in the middle.
On removal from the oven allow to stand for at least 5 mins to set.
Carefully turn out onto cooling wires and allow to cool thoroughly.
When cool they can be sandwiched with jam or curds.
(other fillings can be used)
To finish dust/dredge the top with caster sugar.