Creating the Lemon Blueberry Shortbread Mousse Cake involves several stages. Follow these simple steps for a successful outcome:

Preparing the Shortbread Base

1. Preheat your oven to 350°F (175°C). Grease a 9-inch springform pan.
2. In a bowl, cream together the softened butter and granulated sugar until light and fluffy.
3. Gradually mix in the flour and salt until just combined. Add the vanilla extract and mix until the dough comes together.
4. Press the dough evenly into the bottom of the prepared springform pan.
5. Bake for 25-30 minutes or until the edges are golden brown. Allow it to cool completely in the pan.

Making the Lemon Mousse

6. In a separate bowl, whip the heavy cream with the powdered sugar until soft peaks form. Set aside.
7. In another bowl, whisk together the lemon curd and lemon zest until well combined.
8. In a clean bowl, whip the egg whites with the cream of tartar until stiff peaks form.
9. Gently fold half of the egg whites into the lemon curd mixture, followed by half of the whipped cream. Repeat with the remaining egg whites and whipped cream until well incorporated.

Assembling the Cake

10. Once the shortbread base is completely cool, pour the lemon mousse over the base, spreading it evenly with a spatula.
11. Cover the cake with plastic wrap and refrigerate for at least 2 hours, or until the mousse has set.

Preparing the Blueberry Layer

12. In a small saucepan, combine the fresh blueberries, granulated sugar, lemon juice, and cornstarch. Cook over medium heat, stirring gently, until the mixture begins to boil and thicken.
13. Remove from heat and let it cool completely.
14. Once the mousse has set, carefully pour the blueberry mixture over the top of the lemon mousse layer.

Final Touch

15. Chill the entire cake again for at least another 30 minutes before serving.