1️⃣ Make the sponge cake:
Preheat your oven to 350°F (175°C). Grease and line an 8-inch round cake pan. In a medium bowl, whisk together eggs, sugar, and vanilla until light and fluffy. Sift together flour, baking powder, and salt, then gently fold into the egg mixture. Pour the batter into the prepared pan and bake for 15–18 minutes or until a toothpick inserted into the center comes out clean. Allow the cake to cool completely.

2️⃣ Prepare the blueberry mousse:
In a saucepan, combine blueberries, sugar, and lemon juice over medium heat. Cook until the blueberries break down and release their juice. Puree the mixture and strain through a fine mesh sieve to remove the solids.

3️⃣ Bloom the gelatin:
In a small bowl, dissolve gelatin in 3 tablespoons of water. Let it sit for 5 minutes to bloom. After blooming, gently heat the gelatin mixture until melted, then add it to the blueberry puree.

4️⃣ Whip the cream:
In a separate bowl, whip 1 cup of heavy cream to stiff peaks. Fold the whipped cream into the blueberry mixture, then pour the mousse onto the cooled sponge cake layer in a springform pan. Freeze for at least 4 hours or overnight to set.

5️⃣ Prepare the mirror glaze:
In a saucepan, combine blueberries, sugar, and water over medium heat. Once the blueberries break down, puree the mixture and strain it to remove solids. In a small bowl, bloom the gelatin in ¼ cup water, then heat until melted. Add the gelatin to the blueberry syrup, followed by the heavy cream. Stir until smooth and allow the glaze to cool to 90°F (32°C).

6️⃣ Glaze the cake:
Remove the mousse cake from the springform pan and place it on a cooling rack. Pour the cooled mirror glaze over the mousse cake, ensuring it completely covers the surface. Allow the glaze to set for 30 minutes.

7️⃣ Serve:
Once the glaze is set, transfer the cake to a serving platter and slice to reveal the beautiful layers. Enjoy the creamy, sweet layers of blueberry and mousse!