Grease the bundt tin with butter and dust with flour, tapping off the excess flour.
Mix flour with cornstarch and baking powder.
Peel and core apples, then weigh them.
Grate the apples with a coarse kitchen grater.
Beat softened butter, sugar and salt for about 5 minutes until light and fluffy.
Add eggs one at a time, beating each time until homogeneous.
Add the grated apples as well as cinnamon to the egg butter mixture and mix.
Add the flour mixture and mix just briefly to form a thick batter.
Pour the batter into the baking tin, smooth it out and bake the cake in a preheated oven at 338 °F (170 °C) for about 45 - 50 minutes.
Let the apple bundt cake cool slightly in the baking tin, then carefully turn it out onto a flat plate and let it cool completely.
Dust the cinnamon apple cake with powdered sugar.