Heat the oven to 180°C (350°F), conventional setting. Grease a deep 26 cm (10-inch) springform pan with butter and dust it with fine breadcrumbs or flour, tapping out any excess.

Peel the apples if desired, quarter them, remove the cores, and cut deep parallel slits across the rounded side of each quarter without cutting all the way through. Toss the apples with lemon juice to limit browning.

Cream the softened butter and sugar for about 2–3 minutes, until the mixture looks pale and fluffy. This creates a light base for the tender cake crumb.

Beat in the eggs one at a time, mixing well after each addition, then mix in the vanilla extract or vanilla sugar. Scrape down the bowl so the mixture stays even.

Whisk together the flour, baking powder, and salt in a separate bowl. Add the dry ingredients to the butter mixture and mix only until combined, adding 1–2 tablespoons milk if the batter is unusually stiff.

Spread the thick, spreadable batter evenly in the prepared springform pan. Arrange the apple quarters cut side up over the surface and press them gently into the batter.

Mix the cinnamon and sugar, then sprinkle the mixture evenly over the apples. Add the sliced almonds if using them.

Bake for 45–55 minutes, until the cake is golden brown and a skewer inserted into the cake portion comes out clean. The apples should be tender but still hold their shape rather than collapsing.

Cool the cake in the pan for 15–20 minutes, then carefully remove the springform ring. Continue cooling for at least 20–30 minutes overall before slicing, and serve slightly warm or at room temperature.