Make the Sponge Cake:

1. Prepare the Oven and Pan: Preheat your oven to 350°F (175°C). Line a 9-inch round cake pan with parchment paper and lightly grease it.
2. Whisk the Eggs: In a large bowl, beat the eggs with the sugar and vanilla extract until pale and fluffy, about 5-7 minutes.
3. Combine the Dry Ingredients: In a separate bowl, whisk together the flour, baking powder, and salt.
4. Fold in the Dry Ingredients: Gently fold the dry ingredients into the egg mixture using a spatula, being careful not to deflate the batter.
5. Bake: Pour the batter into the prepared pan and bake for 15-20 minutes, or until the cake is golden and a toothpick inserted into the center comes out clean.
6. Cool the Cake: Let the cake cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.

Prepare the Mousse Filling:

7. Bloom the Gelatin: In a small bowl, sprinkle the gelatin over the water and let it sit for 5 minutes to bloom. Once bloomed, heat it gently in the microwave or over a double boiler until completely dissolved.
8. Whip the Cream: In a large bowl, whip the heavy cream with the powdered sugar and vanilla extract until soft peaks form.
9. Mix Mascarpone and White Chocolate: In a separate bowl, whisk the mascarpone cheese until smooth. If using, fold in the melted white chocolate until well combined.
10. Combine with Gelatin: Once the gelatin has cooled slightly, fold it into the mascarpone mixture. Then gently fold in the whipped cream until the mousse is smooth and fluffy.

Assemble the Charlotte:

11. Prepare the Cake: Once the sponge cake has cooled, slice it into thin layers. You can cut the cake into circles or strips to line the sides of a springform pan or a deep bowl.
12. Line the Pan: Line the sides of the pan with the cake slices, arranging them to form a “wall” around the edges.
13. Fill with Mousse: Pour the mousse filling into the center of the cake-lined pan, smoothing the top with a spatula. Be sure to press the mousse gently into the edges.
14. Chill: Refrigerate the charlotte for at least 3 hours, or until the mousse is set.

Decorate and Serve:

15. Garnish: Once the charlotte is set, carefully remove it from the pan and place it on a serving platter. Dust with powdered sugar and decorate with fresh mint leaves, edible flowers, or fresh cranberries.
16. Serve: Slice and serve the charlotte chilled, enjoying the light, creamy texture and festive appearance.