Preheat the oven to 350°F. Butter an 8-inch cake round.
In a medium bowl, whisk together the flour, cinnamon, and salt.
In the bowl of an electric mixer fitted with the paddle attachment, beat the ½ cup of sugar and the melted butter until it has lightened.
Add the eggs, one at a time, beating between each addition and scraping down the bowl as needed.
Reduce the mixer speed to low and add the flour mixture alternately with the yoghurt, beginning and ending with the flour.
Stir in the sliced apples and the lemon zest, and pour the batter in the prepared pan.
Sprinkle the ⅓ cup sugar evenly over the top of the cake.
The original recipe suggests a baking time of 40 minutes, but I think it needs to bake for over one hour (until a tester inserted into the center (not through an apple if possible!) comes out clean.
Let cool completely before serving.