As first thing, make the custard, you’ll need it to be cold when you will add it to the pound cake. Heat the milk in a saucepan with the peel of one lemon until it starts to simmer, whisk the egg yolks with the sugar and flour, then pour in the hot milk in a thin stream, stirring continuously with a whisk. Put the custard on a low flame and stir constantly until it begins to thicken: remove it from the heat, stir in the grated peel of the second lemon and spoon the custard on a large tray, so that it will cool down quickly.
Preheat oven to 180°C.
Weigh the eggs with their shell. What will be the weight that should be used for flour, yoghurt and sugar.
Beat the eggs and the raw cane sugar until the mixture becomes light and fluffy.
Now add the white yoghurt and stir thoroughly.
Mix in the flour, the baking powder and the cocoa powder. Stir until everything is evenly mixed.
Grease a regular cake mould and dust it lightly with flour.
Scoop half of the cake batter into the prepared mould. Now spread about one third of the lemon custard over the batter, staying clear of the sides of the cake mould. Add the rest of the batter and smooth the top until level, then plop big spoonfuls of the remaining custard on top of the cake, staying clear of the sides. You'll have custard leftover. Drag a butter knife through the custard in a loop-de-loop motion so the cake batter swirls with the custard to create a marbled effect.
Bake for about 40 minutes and let it cool down completely before unmould: do not overbake it, the cake is supposed to be moist in the centre.
Dust with cocoa powder before and serve it with the leftover lemon custard.