Melt the butter in the microwave and leave to cool. Beat the eggs with the sugar until frothy and fluffy (it will only take a few minutes). Now add the yoghurt and the cold butter, continuing to mix with the whisk. Now add the flour, which has been well sifted with the baking powder, while continuing to mix with the whisk. Your dough is ready: pour it into a well buttered and floured cake tin, then add jam to taste all over the surface of the dough. To create the ‘marbled’ effect, simply use a toothpick to create ‘circular patterns’. Bake the cake at 180 degrees, static oven, preheated, for about 40 minutes. Remember to always test the cake with a toothpick before taking it out of the oven.