Beat the eggs with the sugar in a bowl until creamy, add the yoghurt and oil, stir and add the sifted flour, ground walnuts and baking powder.
Stir with a wooden spoon/spatula until the dry ingredients are fully absorbed. The result is a cake mixture, which we're going to be using as a filling.
Divide up the phyllo pastry sheets into 2 piles, with the same amount in each. Smear each of the sheets in the 1st pile with butter and arrange them one atop the other. Repeat this process with the 2nd pile.
Smear an oven dish with a little oil. Put the sheets from 1 pile in it, pour on the cake mixture and place the 2nd pile of sheets on top. Bake in a preheated 338°F (170 °C) oven about 40-60 min. and leave it to cool.
Leave it to cool, then put it in the fridge for at least 6 hours to soften and absorb the syrup.
To make sure that the filling inside isn't raw, check it with a toothpick like you would a regular cake. If it comes out dry, it's ready.