Preheat the oven to 350°F (180°C). Line the bottom of a 9-inch round pan or 9 × 9-inch square pan with parchment paper and lightly grease the sides with butter.
Peel and core the apples, then cut them into thin slices.
Beat the eggs with 160 g sugar, vanilla sugar (or vanilla extract), and salt for 7–8 minutes, until pale, thick, and fluffy.
Sift the flour with the baking powder. Gently fold it into the egg mixture with a spatula, working from the bottom upward.
Pour half of the batter into the prepared pan.
Arrange the apples over the batter. Sprinkle with 1/2 teaspoon cinnamon and half of the sliced almonds.
Pour the remaining batter over the apples and gently spread it evenly.
Mix the remaining 1/2 teaspoon cinnamon with 1 1/2 teaspoons sugar. Sprinkle it over the cake and add the remaining almonds.
Bake for about 30 minutes, until golden and a skewer inserted into an area of batter comes out dry.
Let the cake rest in the pan for 10–15 minutes, then carefully transfer it to a board or wire rack. Cool slightly before slicing.