Step 1: Preheat your oven to 350°F (175°C). Grease and line a 9-inch springform pan with parchment paper.
Step 2: In a food processor, pulse together the all-purpose flour, powdered sugar, unsalted butter, and salt until the mixture resembles coarse crumbs.
Step 3: Press the mixture evenly into the bottom of the prepared springform pan.
Step 4: Bake the crust in the preheated oven for 15-18 minutes, or until lightly golden brown. Remove from the oven and let it cool completely.
For the Lemon Mousse:
Step 1: In a small bowl, sprinkle the gelatin powder over the water and let it bloom for 5 minutes.
Step 2: In a saucepan, heat the fresh lemon juice over low heat until warm. Remove from heat and stir in the bloomed gelatin until dissolved. Let the mixture cool to room temperature.
Step 3: In a mixing bowl, beat the heavy cream, granulated sugar, and lemon zest until stiff peaks form.
Step 4: In another mixing bowl, beat the softened cream cheese until smooth.
Step 5: Gradually add the cooled lemon juice mixture to the cream cheese, beating until smooth and well combined.
Step 6: Gently fold the whipped cream into the cream cheese mixture until fully incorporated.
For the Blueberry Compote:
Step 1: In a saucepan, combine the fresh blueberries, granulated sugar, and fresh lemon juice.
Step 2: Cook the mixture over medium heat, stirring occasionally, until the blueberries soften and release their juices, about 5-7 minutes. Remove from heat and let it cool to room temperature.
Assembling the Mousse Cake:
Step 1: Once the shortbread crust has cooled, spread a layer of the lemon mousse evenly over the crust.
Step 2: Spoon the blueberry compote over the lemon mousse layer, spreading it out evenly.
Step 3: Repeat the layering process with the remaining lemon mousse and blueberry compote, finishing with a layer of blueberry compote on top.
Step 4: Refrigerate the mousse cake for at least 4 hours, or until set.
Step 5: Before serving, run a knife around the edge of the springform pan to loosen the cake. Carefully remove the sides of the pan.
Step 6: Slice the Lemon Blueberry Shortbread Mousse Cake into wedges and serve chilled.
