This sponge mixture is flavours with bi-tua leaves to give a green colour and bubble gum like flavour, so typical of bi-tua dishes. It's better to use bi-tua flavouring for this because it's concentated, but if you cannot get it, shred bi-tua in water and squeeze the leaves carefully to get as concentrated a flavour as possible into the water, then filter out the pulp so that only the green water is used.
To make a more interesting photograph, I've cut the sponge into cubes, but that is optional.

Ingredients
170 gms Unsalted Butter
170 gms Castor Sugar
4 Large Eggs
160 gms Cake Flour (With raising agent)
4-5 Drops Bi Tua Flavour

Preparation
1. Leave the butter to soften at room temperature.
2. Blend the butter with the sugar to form a smooth creamy mixture.
3. Add 2 of the eggs and whisk in until smooth, it's better to use an electric whisk to get as much air into the mixture as possible.
4. Add the remaining eggs and whisk again.
5. Add the flour a little at a time, whisking in until smooth.
6. Finally finish by adding the bi-tua flavour. If the mixture is too loose, add a little more flour, you want a soft creamy paste at the end.
7. Line a cake dish with greaseproof paper, pour in the mixture. Bake in a pre-heated oven for 45-50 minutes at 180 degrees celsius.