In a large bowl, whisk the eggs and granulated sugar together until pale and fluffy.
Sift the flour and baking powder together, then gently fold into the egg mixture until just combined.
Pour the sponge cake batter into a greased and floured cake pan. Bake in a preheated oven at 350°F (180°C) for 20-25 minutes, or until a toothpick inserted into the center comes out clean. Let cool completely.
Soak the gelatin sheets in cold water until softened.
Heat the blueberry puree in a saucepan over low heat. Stir in the powdered sugar and vanilla extract. Squeeze the excess water from the softened gelatin sheets and add them to the warm puree. Stir until completely dissolved. Let cool slightly.
Whip the heavy cream until stiff peaks form. Gently fold the cooled blueberry puree into the whipped cream to create a light and airy mousse.
Slice the cooled sponge cake in half horizontally. Spread the blueberry mousse evenly between the layers. Frost the top of the cake with the remaining mousse, smoothing it out.
Decorate the cake with fresh blueberries. Refrigerate for at least 4 hours, or preferably overnight, to allow the mousse to set and the flavors to meld.
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