Prepare the syrup well before you start making the yaourtopita as it should be completely cold when ready to pour. Pour the sugar and the water into a saucepan along with the vanilla, stir until it dissolves, add the lemon zest and bring to a boil for 7΄-8΄. Set the syrup aside to cool.
Combine the two types of semolina with the baking powder and the baking soda. Butter a 49 cm diameter baking tray. Beat the sugar with the butter in a mixer at high speed until fluffy. Add the eggs one by one and then gradually spoon in the tubs of yogurt (along with the skins).
Next, add the lemon zest, the vanilla and the cognac and, finally, turning the mixer down to low, scoop in the semolina mixture. Spread the mixture on the buttered baking tray and bake in a pre-heated oven at a medium (180 °C) heat for approximately 50΄-60΄ to 1 hour, depending on your oven.