First Step: MAKE THE SHORTBREAD CRUST

1. Preheat the Oven: Preheat your oven to 350°F (175°C). Prepare a 9-inch springform pan by lining the bottom with parchment paper. This will ensure easy removal of the cake after it’s baked and chilled.
2. Prepare the Shortbread Dough: In a medium mixing bowl, combine the all-purpose flour, powdered sugar, and a pinch of salt. Add the cold, cubed butter and use a pastry cutter or your fingers to work the butter into the dry ingredients until the mixture resembles coarse crumbs. You can also use a food processor for this step if you prefer.
3. Press into the Pan: Once the dough has come together, press it evenly into the bottom of the prepared springform pan. Use the back of a spoon to gently pack it down.
4. Bake the Crust: Place the pan in the oven and bake for 12-15 minutes, or until the crust is lightly golden around the edges. Remove from the oven and allow the crust to cool completely while you prepare the mousse.

Second Step: MAKE THE LEMON BLUEBERRY MOUSSE

1. Whip the Cream: In a chilled mixing bowl, whip the heavy cream with an electric mixer until it forms soft peaks. Be careful not to overwhip the cream. Set aside.
2. Prepare the Gelatin: In a small bowl, combine the gelatin with 2 tablespoons of cold water. Let it sit for about 5 minutes to bloom. After blooming, gently heat the gelatin mixture in the microwave for 10-15 seconds until it dissolves completely. Set it aside to cool slightly.
3. Combine the Mascarpone, Sugar, and Lemon: In a separate large bowl, whisk together the mascarpone cheese, granulated sugar, lemon zest, and fresh lemon juice until smooth and creamy.
4. Add the Gelatin: Slowly pour the slightly cooled gelatin mixture into the mascarpone mixture, whisking constantly to prevent any lumps. The gelatin helps the mousse set and gives it a firm yet airy texture.
5. Fold in the Whipped Cream: Gently fold the whipped cream into the mascarpone mixture. Do this carefully to maintain the mousse’s light texture.
6. Add the Blueberries: Gently fold in the fresh blueberries and blueberry jam. This will give the mousse a lovely swirl of color and flavor. Be sure not to overmix, so the blueberries remain whole and evenly distributed.
7. Fill the Pan: Once the mousse is prepared, pour it on top of the cooled shortbread crust. Use a spatula to smooth the top evenly. Refrigerate the cake for at least 3 hours, or until the mousse is fully set.

Third Step: MAKE THE BLUEBERRY TOPPING

1. Prepare the Simple Syrup: In a small saucepan, combine the fresh blueberries, lemon juice, and sugar. Cook over low heat, stirring occasionally, until the blueberries begin to release their juices and the sugar dissolves, creating a simple syrup. Let it cool to room temperature.
2. Top the Cake: Once the mousse has set and is firm, spoon the cooled blueberry syrup and fresh blueberries on top of the mousse layer. Spread the blueberries evenly over the top, allowing the syrup to seep into the mousse for extra flavor.
3. Chill Again: Return the cake to the refrigerator for an additional 30 minutes to an hour to ensure the topping sets and the flavors meld.

Final Step: GARNISH AND SERVE

1. Add the Final Touches: Before serving, you can garnish the cake with a sprinkle of lemon zest and a few fresh mint leaves for a pop of color and fragrance.
2. Remove from the Pan: Carefully release the sides of the springform pan and transfer the cake to a serving platter. Slice into wedges, making sure to cut through the shortbread crust and mousse layers evenly.
3. Serve and Enjoy: Serve the Lemon Blueberry Shortbread Mousse Cake chilled for a refreshing and indulgent treat. The combination of creamy mousse, sweet blueberries, and buttery shortbread will leave your guests asking for seconds!