Before starting:
Place the oven rack in the center and preheat the oven to 350F (180C).
Butter 2 round molds of 9″X 2″ (23cm in diameter and 5cm high) and line the bottom with buttered parchment paper.
Place the molds on a baking sheet.

Make the cake:
Sift the flour, baking powder, and salt.
Whisk together the milk and egg whites in a medium bowl.
Place the sugar and zest in a large bowl or the bowl of a KitchenAid and rub between your fingers until the sugar is moist and fragrant.
Add the butter and with the KitchenAid paddle or an electric mixer, beat at medium speed for 3 minutes, until the butter and sugar are light and fluffy.

Add the vanilla and one-third of the flour, continuing to beat at medium speed.
Add half of the milk-egg mixture, then half of the remaining flour.
Add the rest of the milk-egg mixture and beat well, then add the remaining flour to finish.
Finally, beat for 2 minutes to aerate the dough well.

Divide the dough between the two molds and smooth the top with a rubber spatula.
Bake for 30-35 minutes, performing the toothpick test which comes out clean from the center.
Place the molds on a wire rack and let cool for 5 minutes, run a knife around, turn over, and remove the paper.
Turn the cake over and let cool to room temperature. (When cool, it can be wrapped and stored at room temperature overnight or frozen for up to 2 months)

For the buttercream:
In a stainless steel bowl, beat the eggs, sugar, and orange liqueur with a whisk.
Heat the mixture over a bain-marie and beat with an electric mixer until the mixture reaches 65C.
As soon as the mixture is at the right temperature, remove the bowl from the heat and mix with an electric mixer until cooled.
Add the softened butter and continue mixing with a whisk.

For the fondant:
Place the mini marshmallows and water in a large microwave-safe bowl.
Microwave on high for 2 minutes (if the marshmallows overflow, stir the contents of the bowl once during the process).

In the mixer bowl, fitted with the triangular hook, add a little powdered sugar, then pour in the melted marshmallows.
Mix until the powdered sugar is incorporated, then add the remaining powdered sugar, a little at a time, until it is well incorporated and the mixture is smooth and comes away from the sides of the bowl.

Remove the fondant from the bowl, wrap it in plastic wrap, and refrigerate overnight.

To color the fondant, wearing latex gloves, press a finger into the center of the mass and add a few drops of coloring, close the fondant over and knead the dough between your hands (or on a surface dusted with powdered sugar) until the color is uniform.

To roll out the fondant, use a pastry roller and cornstarch so it doesn’t stick to the work surface.

Assembly:
With a sharp knife, cut each cake in half horizontally.
Place the cut side up on a cake board or a protected plate.
Pour 1/3 of the jam and cover with a quarter of the buttercream.
Cover with half of the cake and repeat the operation once.
Place the last layer with the cut side down and use the remaining buttercream to glue the sides and top.