Preheat the oven to 325°F (170°C) and lightly grease an 8-inch (21 cm) spring form tin with butter.
Peel, core, and halve the apples. Set each cored-side down on a cutting board and place a chopstick or wooden spoon on each side of the apple. Cut thin parallel slices with a knife (the chopsticks will stop it from cutting all the way through) to hasselback the apples.
4-6 medium apples*
Add the butter and sugar to a large mixing bowl or the bowl of a stand mixer. Beat on high speed for 4-5 minutes, or until creamy and light.
⅔ cup salted butter**, ¾ cup sugar
Add the eggs one at a time, beating between each addition, until well incorporated. Beat in the vanilla.
3 large eggs, 1 teaspoon vanilla extract
Add half of the flour and all of the baking powder. Mix on low speed until combined.
2 cups all-purpose white flour, 2 tablespoons baking powder
Mix in the milk, then add the remaining flour and mix until no streaks of flour remain. Don’t over-mix.
Scant ¼ cup whole milk
Bake for 50-55 minutes, or until golden and the cake passes a skewer test. Cool for 10 minutes in the tin before removing the sides and cooling fully on a wire rack. Serve with whipped cream.
Notes
* depending on size. Apples should mostly cover the top of the cake.** if using unsalted butter, add a half teaspoon of salt with the flour.Added vanilla is not traditional but is nice.