Begin by separating the eggs into two bowls: place the egg whites in one and the yolks in the other.

In the bowl with the egg whites, add ¼ teaspoon of salt and ½ cup of sugar. Beat the mixture until stiff peaks form and set aside for later use.

In a large mixing bowl, combine the softened butter, remaining sugar, vanilla sugar or extract, lemon extract, and rum. Beat this mixture until it becomes light and fluffy.

Gradually incorporate the egg yolks into the butter mixture, mixing at a low speed to ensure a smooth blend.

In a separate bowl, sift together the flour, cornstarch, and baking powder to ensure they are evenly mixed.

Slowly warm the milk until it is lukewarm, being careful not to overheat.

Alternate between adding the sifted dry ingredients and the lukewarm milk to the egg yolk mixture, gently folding in the stiffened egg whites as well. This can be done by hand or with a mixer set to the lowest speed to avoid overmixing.

Grease a cake pan thoroughly to prevent sticking.

Pour about two-thirds of the batter into the prepared cake pan.

Sift the cocoa powder into the remaining batter and mix well until the color is consistent.

Spoon the chocolate batter over the plain batter in the cake pan. To achieve the marble effect, gently swirl a fork through the two layers of batter.

Bake the cake at 350°F (175°C) for 40 to 50 minutes, or until a toothpick inserted in the center comes out clean.

Allow the cake to cool in the pan for approximately 15 minutes before removing it to cool completely on a wire rack.

Once the cake is fully cooled, finish by dusting with powdered sugar or coating with a rich chocolate glaze, depending on your preference.