Place a rack in the center of the oven and preheat to 375 degrees f.
Grease and flour a 9-inch spring-form pan.
In a large bowl, add salt, butter, 2 cups flour, and ½ cup sugar.
Cut in butter with your fingers or with a pastry blender until crumbly.
Remove 1 cup of the crumbs and press them over the bottom and about ½ inch up the sides of the spring-form pan, making a crust about ⅛ inch thick.
Make a well in the remaining crumb mixture.
Add the milk, eggs, vanilla, lemon zest, the remaining 3 tbsp.
flour, and the baking powder.
With a whisk, blend this mixture thoroughly, without incorporating the crumbs.
Then, with a wooden spoon stir in the crumbs until well blended but still a little lumpy.
Core, peel and chop apples into ½ inch pieces, then fold the apples into the mixture.
Scrape the batter into the pan.
In a small bowl, beat the egg white until foamy.
Beat in the remaining 2 tbsp sugar and beat until the whites barely stand in peaks.
Spread over the top of the batter.
Bake 65–75 minutes or until a knife inserted in the center comes out clean.
Remove from the oven and cool 30 minutes on a wire rack.
Slip off the sides of the pan and finish cooling the cake.
Serve at room temperature.
Covered in plastic wrap, the cake holds well at cool room temperature up to 2 days.
It keeps a week in the refrigerator.
Either way, warming it in the oven makes it even tastier.
Easy to move around and heat if you leave it on the pan base (when cold, put in a cold oven on a tray, set temperature to 350°F. Turn off oven when temperature reaches 350°F).
Delicious served alone, but one could add whipped cream.