Soak gelatin sheets in a small bowl filled with cold water. Prepare lemon syrup. Add sugar, lemon zest, lemon juice, Limoncello, and vanilla paste to a saucepan. Place over medium-high heat and bring to a boil, then cook for 3 - 4 minutes. Remove from the heat and set aside.
cookies and lemon cream
Arrange Amaretti cookies at the bottom of each serving bowl, you will need four bowls. Drizzle 1 tablespoon of the lemon syrup over the cookies and set aside. Add soaked and drained gelatin sheets to the rest of the lemon syrup in a saucepan. Stir, so that the gelatin dissolves completely. Add greek yogurt and stir well to incorporate. Let the mixture cool to room temperature.
Add cold whipping cream and sugar to a large bowl. Whip the cream until stiff peaks form. Gradually (in three additions) gently fold the lemon - yogurt cream to the whipped cream. You will get a creamy, light citrusy mousse.
Pour the mousse over the Amaretti cookies in serving bowls. Place in the fridge for 2 - 6 hours, or even better, overnight. The mousse has to firm and become super creamy. Decorate with lemon zest and serve.