Preheat the oven to 350°F. Grease a 9-inch springform pan with cooking spray and set aside. In a food processor, pulse the Nilla wafers until they form fine crumbs. Add in the melted butter and process until well combined.
Transfer the mixture to the prepared pan and spread it out evenly. Use a measuring cup to press the crust firmly into place. Bake for 15 minutes or until golden brown. Let cool while you prepare the mousse.
In a small saucepan, heat the blueberries and lemon juice over low heat until softened and fragrant. Purée until smooth and set aside to cool.
Add the unflavored gelatin to a heatproof bowl and pour in 2 tablespoons of cold water. Let stand for 5 minutes until the gelatin is completely dissolved. Place the white chocolate chips in a food processor and process until very fine crumbs form. Transfer into a medium bowl, add the lemon zest, and set aside.
In a large bowl, whip the heavy cream and powdered sugar together until soft peaks form. Gently fold in the blueberry purée and white chocolate mixture into the whipped cream until fully incorporated.
Add the cooled gelatin and mix until evenly distributed. Pour the mousse over the crust and spread it to even out the top. Refrigerate for at least 4 hours, or until set.
For the topping, combine the blueberries, sugar, and lemon juice in a small saucepan over low heat. Cook until the mixture has thickened into syrup. Let cool to room temperature before serving.
Carefully remove the springform pan and transfer the cake to a serving platter. Drizzle with the blueberry topping and serve chilled.