I make marble cake quite often, here is the recipe.
225 grams stork
225 grams golden granulated sugar
2 tbs vanilla extract
225 self raising floor (or plain floor + 2 tsp baking powder)
50 gr cocoa powder
I use Stork instead of butter, and first thing to do is take it out of the fridge. It is quite soft so doesn't need to wait outside for long. 10 minutes is usually enough.
While Stork is getting to room temperature, heat the oven to gas mark 5 and line a 20cm round cake tin first.
Then cream 225 grams of stork with electric food mixer in a bowl and add 225 grams of golden caster sugar to it and mix until light and fluffy.
In an other bowl, whisk 4 eggs with 2 table spoons of vanilla extract then add this to Stork and sugar mixture slowly while continuing to beat the mixture.
Continue to mix with electric mixer until everything is light and there is quite a lot of air in the mixture, this makes the final cake light.
Sift 225 grams of self raising flour (or plain flour and 2 teaspoon baking powder) in a bowl and then sift for the second time into the cake mixture. With a spoon slowly fold flour taking care not to knock the air out of the cake mixture. Sifting the floor twice makes the cake even more lighter!
Tip half of the cake mixture in an other bowl and sift 50 grams of cocoa powder into this half and fold in with a spoon taking care not to knock the air out of the mixture.
Take spoonful of each cake mixture and place in the prepared tin alternating the vanilla with the cocoa. When finished tap the tin on the work floor lightly so everything settles in. Then with a skewer swirl the mixtures to give the marble effect.
Bake in the oven for 50 minutes or when a skewer inserted in the cake comes out clean.
If I have limes or oranges at home sometimes I bake "lime syrup marble cake" Dan Lepard recipe, mostly replacing the lime with oranges.