For the Sponge Cake:

1. Preheat your oven to 180°C (350°F) and line a 12×16-inch baking tray with parchment paper.
2. In a mixing bowl, beat the eggs, caster sugar, and vanilla extract until the mixture is pale, fluffy, and doubled in volume. This will take about 5-7 minutes.
3. Sift the all-purpose flour, baking powder, and salt into the egg mixture. Gently fold them in using a spatula, being careful not to deflate the batter.
4. Pour the batter onto the prepared baking tray and spread it evenly.
5. Bake for 10-12 minutes until the sponge cake is lightly golden and springs back when touched.
6. Remove the cake from the oven, let it cool for a few minutes, and then transfer it to a clean kitchen towel dusted with icing sugar. Roll the cake tightly with the towel and allow it to cool completely.

For the Mango Filling:

1. In a small bowl, sprinkle the gelatin over cold water and let it bloom for 5 minutes.
2. In a blender, puree the diced mangoes until smooth. Add lemon juice and mix well.
3. In a saucepan, heat the mango puree and granulated sugar over low heat until the sugar dissolves.
4. Remove the saucepan from heat, add the bloomed gelatin, and stir until it’s completely dissolved and incorporated.
5. Let the mango mixture cool to room temperature.
6. In a separate bowl, whip the heavy cream until stiff peaks form.
7. Gently fold the whipped cream into the mango mixture until well combined.

Assembly:

1. Carefully unroll the cooled sponge cake from the kitchen towel.
2. Spread the mango filling evenly over the cake, leaving a small border around the edges.
3. Roll the cake back up, using the parchment paper to help you.
4. Place the rolled roulade on a serving platter, seam-side down.
5. Decorate the top with additional mango slices and fresh mint leaves if desired.
6. Chill the Swiss Mango Roulade in the refrigerator for at least 1-2 hours before serving.