The portuguese often make light sponges which is why the classic Madeira cake has come to be called as such. To add a twist they will often marble a vanilla and chocolate mix.
Translated for you, I originally got this from my granny so had to make it and figure out how I could write this method best as her expertise means that the version she gave me was short and sweet. Here it is:
Ingredients

5 eggs – separated
300g sugar
400g plain flour
250g butter (or margarine)
1 &1/2 cups of milk (125ml)
1 Tablespoon Royal Baking Powder
1 Tablespoon cocoa powder

Method

1. Cream the sugar and butter together until fully incorporated.
2. Add the egg yolks.
3. Add the flour, milk and baking powder.
4. Beat egg whites separately until they are hard and fold into the rest of the batter.
5. Divide the mixure/ separate about half into another bowl.
6. Add the cocoa powder to one half of the mix – mix well until evenly mixed in.
7. Grease a cake tin and line with flour and take it in turns to pour the ‘vanilla’ and ‘chocolate’ mixtures so that they form a marble effect.

8. Bake for 40 minutes or until crisp on top and a metal or wooden skewer comes out clean when you test it.

The original from my granny…
5 ovos, 300gr. açúcar, 400gr.farinha, 250gr.manteiga ou margarina, 1 e meia chávena de leite, 1 colher sopa de Royal Baking Powder, um pouco de chocolate.
Bate-se o açúcar com a margarina depois junta-se as gemas seguindo-se a farinha, o leite e o fermento.
Á parte batem-se as claras em castelo que se junta a toda a massa.
Divide-se a massa e numa parte junta~se o chocolate em pó. Deita-se numa forma untada e polvilhada com farinha às camadas e vai ao forno.
Bom apetite e que vos saiba muito bem.