Preheat oven to 350°F (175°C) and grease the bottom of a 9- or 10-inch springform or use parchment paper.
Peel, quarter, and core the apples. Thinly slice each quarter lengthwise without cutting all the way through (like in the image above).
In the bowl of a stand mixer fitted with the paddle attachment, beat butter until creamy, about 1 minute. Add sugar and mix until light and fluffy, about 2 minutes.
Add eggs one at a time and beat for 30 seconds on high speed after each addition.
Add vanilla sugar (or extract) and lemon juice. Beat until combined.
Combine cake flour, baking powder, and salt. With the mixer running on low speed add half of the flour, then 1 tbsp milk, followed by the remaining flour, and then the remaining milk. Beat until just combined. Don't overmix!
Transfer batter into prepared pan and smooth the top with a spatula. Distribute the apples with the sliced side up on top (see image above).
Bake in the lower third of the oven for 45 minutes, until lightly browned and a skewer comes out clean.
Transfer to a cooling rack and remove the springform ring. Let cake cool on the bottom of the springform pan to room temperature.
Sprinkle powdered sugar over the cooled cake and serve with sweetened whipped cream.