Beat the butter till it is properly creamed and then add the sugar and combine well.

Separate the eggs and add the yolks one at a time. Then add half of the milk and all the flour and vanillin.

While continuing stirring, add the rest of the milk where we've already put the cream of tartar and the whites of the eggs beaten to a meringue.

Add the baking soda with the cognac and the zest, last.

Place in a buttered pan and bake for about 1 hour at 200°, depending on your oven. If it is a strong one, bake at 180 and allow a bit more of baking time. Anyway, since every oven is different and baking time also depends on the pan, do the test with the knife and let it in the oven till the knife comes out clean.