Preheat your oven to 350°F (175°C). In a medium bowl, combine the graham cracker crumbs, granulated sugar, and melted butter, mixing until well blended. Press this mixture evenly into the bottom of a 9x13-inch baking dish, ensuring a compact and even layer. Bake for 10 minutes or until the crust is golden brown. Remove from the oven and allow it to cool completely.

In a saucepan, whisk together granulated sugar, fresh lemon juice, and lemon zest. Heat the mixture over medium-low heat until the sugar dissolves, stirring constantly. Gradually whisk in the egg yolks and continue to cook for about 8-10 minutes until thickened. Remove from heat and stir in the butter until smooth. Pour the lemon curd over the cooled crust and refrigerate for at least 2 hours to set.

While the lemon curd chills, prepare the whipped cream. In a large mixing bowl, beat the chilled heavy cream with an electric mixer on medium speed until soft peaks form. Gradually add the elderflower liqueur, sugar, and vanilla extract, then continue beating until stiff peaks form.

Once the lemon curd is set, gently spread the whipped cream over the lemon layer using a spatula for even distribution. Refrigerate the assembled dessert for an additional 30 minutes.

If including a meringue topping, preheat your broiler. In a clean bowl, beat the egg whites on medium-high speed until stiff peaks form. Gradually fold in the remaining sugar and vanilla extract.

Spread or pipe the meringue over the whipped cream layer. Place the dessert under the broiler for a few minutes until the meringue turns golden brown.