Preheat the oven to 350 F (180 C).
Line an 8-inch (20 cm) square Pyrex mold with parchment paper or a 12-muffin tin that you have buttered and floured.
Peel and core the apples, cut them in half.
Thinly slice them with a mandoline and add the lemon juice and mix well so that the apples do not turn brown. Set aside.

Melt the butter in the microwave. Set aside.
Whisk the eggs, sugar, and vanilla seeds in a bowl and whisk vigorously until the mixture whitens and triples in volume.
Pour the melted butter into the preparation, mixing gently until a homogeneous mixture is obtained.
Add half of the milk, mix, then add the sifted flour with the baking powder and whisk the mixture gently until it is homogeneous, then add the rest of the milk and mix.
The batter becomes thick and homogeneous.
Add the lemon apple slices and mix them gently into the batter with a flexible spatula so as not to break them.

Pour the batter into the prepared mold, smoothing the surface with the spatula.
Bake for about 60 minutes for the square mold or 45 minutes for the muffin molds.

Let cool before unmolding.
Sprinkle with powdered sugar.
The cake is easier to cut when it’s cold.