Step 1: In a large bowl, cream together the softened butter and powdered sugar until light and fluffy. Stir in the vanilla extract, then gradually add the all-purpose flour and a pinch of salt until just combined.
Step 2: Press the dough evenly into the bottom of a greased and parchment-lined 9-inch springform pan. Prick the crust all over with a fork. Bake at 350°F (175°C) for 20-22 minutes, or until lightly golden. Let it cool completely on a wire rack.
Step 3: While the crust cools, combine the blueberries, 1/4 cup sugar, and 1 tablespoon lemon juice in a small saucepan over medium heat. Bring to a gentle simmer, stirring occasionally, until the berries begin to burst and the sauce thickens slightly (about 5-7 minutes). Stir in the cornstarch slurry and cook for another minute, until thickened. Remove from heat and let cool completely to room temperature.
Step 4: In a large bowl, beat the softened cream cheese, 1/2 cup sugar, lemon zest, 1/4 cup lemon juice, and 1 teaspoon vanilla extract until smooth and creamy. In a separate, chilled bowl, whip the heavy cream until stiff peaks form. Gently fold the whipped cream into the lemon cream cheese mixture until no streaks remain and the mousse is light and airy.
Step 5: Pour half of the lemon mousse over the cooled shortbread crust, spreading it evenly. Spoon dollops of the cooled blueberry compote over the mousse layer. Using a knife or skewer, gently swirl the compote into the mousse for a marble effect.
Step 6: Carefully pour the remaining mousse over the compote layer and repeat the swirling process with the rest of the blueberry compote.
Step 7: Cover the springform pan loosely with plastic wrap and refrigerate for at least 6 hours, or preferably overnight, to allow the mousse to set completely. Once set, carefully remove the sides of the springform pan. Garnish with fresh blueberries, extra lemon zest, or a sprig of mint if desired. Slice and serve!