Preheat to oven to 350°F or 170°C.
Grease and flour an 8 inch round loose bottom or springform cake pan. I like to line the bottom of the pan with a circle of baking parchment as well.
Sift together the flour, baking powder, salt and 1 teaspoon cinnamon.
Peel, core, quarter and chop one of the apples into small pieces.
Stir the apple pieces into the flour mixture. Set aside.
In an electric mixer, cream together the butter and sugar. Beat in the eggs and vanilla thoroughly.
Using a wooden spoon, stir in the flour and apple mixture.
Add the yogurt and beat together thoroughly.
Pour into the prepared pan, smoothing the top.
Core, quarter and thinly slice the two apples. (Don't peel them!)
Arrange the apple slices on top of the cake in a circular fashion.
Combine the topping ingredients, 2 tablespoons sugar and ½ teaspoon cinnamon.
Sprinkle over the apples on top of the cake.
Bake for 25 to 30 minutes or until a skewer inserted in the centre of the cake comes out clean.
Allow to cook in the pan for at least 20 minutes before carefully removing the cake from the pan and cooling completely on a wire rack.