In a large bowl, vigorously whisk the egg yolks until foamy. Set aside.
In a small saucepan, combine half (100g or ½ cup) of the sugar and water. Place over medium heat and stir frequently until it reaches 240 degrees F.
Slowly drizzle the hot sugar syrup into the egg yolks, while whisking simultaneously. Continue whisking vigorously for 1-2 minutes until the mixture has cooled.
Once cool, whisk in the cold mascarpone until fully combined. Place the bowl in the fridge while you make the meringue.
In a medium heat proof bowl, whisk together the egg whites, salt, and remaining (100g or ½ cup) of sugar. Place the bowl over the pot of simmering water, and whisk frequently until it reaches 120 degrees F.
Remove the bowl from heat and use an electric mixer to beat until stiff peaks form.
Using a rubber spatula, gently fold the meringue into the egg yolk, mascarpone mixture until fully combined.