Prepare the Shortbread Crust : In a bowl, combine 1/2 cup of unsalted butter, 1/4 cup powdered sugar, and a pinch of salt. Mix until smooth. Gradually add 1 cup of flour and knead into a dough. Press the dough into a greased 9-inch springform pan to form an even layer. Chill the crust in the refrigerator for 20 minutes.

Make the Lemon Mousse : In a small bowl, combine 1 tbsp of gelatin powder with 2 tbsp of water. Let it bloom for 5 minutes. Heat the gelatin mixture over low heat until fully dissolved. Set aside to cool slightly. In a mixing bowl, beat the cream cheese, 1/2 cup powdered sugar, and lemon zest until smooth. Gradually add 9/16 cup of fresh lemon juice and the dissolved gelatin. Mix well. In another bowl, whip 1 cup of heavy cream until stiff peaks form, then gently fold it into the lemon mixture.

Prepare the Blueberry Topping : In a saucepan, combine 2 cups of fresh blueberries, 3/4 cup granulated sugar, and 2 tbsp of water. Cook over medium heat until the blueberries soften and the mixture thickens. Let the mixture cool to room temperature.

Assemble the Cake : Spread the lemon mousse evenly over the shortbread crust. Pour the cooled blueberry mixture on top of the mousse layer, spreading it out evenly. Refrigerate the cake for at least 4 hours or overnight to set.

Serve and Enjoy : Carefully remove the cake from the springform pan. Garnish with additional lemon zest and fresh blueberries if desired. Slice and serve chilled. Enjoy the refreshing combination of lemon and blueberry in every bite!