In a large bowl, cream the butter, then add the milk, mix well. Add one by one the egg yolks beating well after each addition.
In another bowl, sift together the flour with the cinnamon and the baking powder, set aside.
In a bowl, whisk the egg whites until foamy. Add little by little the sugar and whisk until getting stiff peaks.
Add the egg white mixture to the egg yolk mixture, just beating until blended.
Add little by little the flour mixture just mixing until blended.
Pour in a greased baking pan and bake in a previously pre-heated oven at 180°C for 40 minutes or until a toothpick inserted in the center comes out clean.