Prepare the Shortbread Crust:

• Preheat your oven to 350°F (175°C). Grease and line a 9-inch springform pan with parchment paper.
• In a large bowl, cream together the softened butter and granulated sugar using an electric mixer until the mixture is light and fluffy, about 3-5 minutes.
• Gradually add the all-purpose flour and salt to the butter mixture. Mix until the dough starts to come together, being careful not to overmix.
• Press the shortbread dough evenly into the bottom of the prepared pan. Prick the surface with a fork to prevent it from puffing up during baking.
• Bake in the preheated oven for 20-25 minutes, or until the edges are lightly golden. Once baked, remove from the oven and allow the crust to cool completely in the pan.

2. Make the Lemon Mousse:

• In a medium mixing bowl, whip the heavy cream using an electric mixer until soft peaks form.
• Gradually add the powdered sugar while continuing to whip until stiff peaks form.
• In another bowl, combine the freshly squeezed lemon juice and lemon zest. If using gelatin, dissolve it in a tablespoon of warm water and mix it into the lemon juice.
• Gently fold the lemon mixture into the whipped cream until fully incorporated, taking care not to deflate the mousse.

3. Prepare the Blueberry Layer:

• In a medium saucepan, combine the fresh blueberries, granulated sugar, cornstarch, water, and lemon juice. Cook over medium heat, stirring gently, until the mixture begins to thicken and bubble, about 5-7 minutes.
• Remove from heat and let the blueberry mixture cool slightly.

4. Assemble the Cake:

• Once the shortbread crust has cooled, pour the lemon mousse over the crust, smoothing the top with a spatula.
• After the mousse has set for about 1 hour, spoon the blueberry mixture on top, spreading it evenly.
• Cover the cake with plastic wrap or aluminum foil and refrigerate for at least 4 hours, or overnight for the best results.

5. Serve:

• Before serving, carefully release the springform pan and remove the cake. Garnish with fresh blueberries, lemon slices, and mint leaves if desired.
• Slice and enjoy your refreshing Lemon Blueberry Shortbread Mousse Cake!