Fun Fact: According to Irish folklore, it is said that St. Patrick himself planted an apple tree in an ancient settlement outside Armagh city. Today, County Armagh is known as "Orchard County" and celebrates its apples with festivals and apple-blossom tours.
Preheat oven to 350 F and grease a 9-inch round springform cake pan.
To make streusel topping, sift flour into a bowl and rub in the butter until the mixture resembles bread crumbs. Stir in the sugar and reserve.
Peel, core and thinly slice the apples. To make the cake, sift the flour into a bowl with the baking powder, cinnamon and salt. Place the butter and the superfine sugar in a separate bowl and beat together until light and fluffy. Gradually beat in the eggs, adding a little of the flour mixture with the last addition of egg. Gently fold in half of the remaining flour mixture, then fold in the rest with the milk.
Spoon the batter into the prepared springform pan and smooth the top. Cover with sliced apples and sprinkle with streusel topping evenly over top. Bake in the preheated oven for 1 hour, or until browned and firm to the touch. Let cool IN THE PAN before removing the sides. Dust the cake with confectioners' sugar before serving.