Preheat oven to 180C. Line a baking tray with baking paper.
For the Biscuit Base, place butter and chocolate in a medium, heat proof bowl and place over a saucepan of simmering water, stirring frequently until mixture has melted. Remove from heat and allow to cool slightly.
Meanwhile, cream egg and sugar in a bowl of an electric mixer until light and fluffy. Once creamed, whisk in cooled chocolate and butter mixture, then fold through the flour and baking powder.
Spread the mixture onto the prepared baking tray to a thickness of 5mm. Bake for 10-12 minutes until golden. Remove from oven and set aside to cool.
For the Parfait, place egg white and sugar in the bowl of an electric mixer fitted with a whisk attachment and whip until thick and glossy, about 10 minutes.
In a separate bowl, whisk cream to soft peaks. Fold the whipped egg white along with the rose water, white chocolate and rose petals into the whipped cream.
Spread onto a metal baking tray to a thickness of 1cm. Transfer to freezer until firm.
For the Marshmallow, first place gelatine in a bowl of cold water and set aside to bloom.
Place glucose and sugar in a small saucepan over medium heat and bring to a boil. Turn down heat and simmer until sugar has dissolved and mixture has reduced a little. Set aside to cool slightly until needed.
Meanwhile, place egg whites in the bowl of an electric mixer and whisk until soft peaks form. With the motor running, pour ¾ of the sugar syrup into the whisked egg whites and continue to whisk until mixture is thick and glossy.
Remove the softened gelatine leaves from the water and squeeze to remove excess moisture. Add to remaining sugar syrup and stir to ensure the gelatine is fully dissolved.. Once melted, whisk into the marshmallow mixture and continue to beat until cooled slightly. Transfer to a piping bag and set aside until needed.
For the White Chocolate Domes, place 150g of the white chocolate in a heat proof bowl and place over a saucepan of simmering water, stirring frequently until chocolate melts. Once melted, pour the chocolate evenly between 10 x 6cm silicon moulds. Flip the moulds over to allow excess chocolate to drain off. Scrape edges of the moulds with a palette knife to neaten, then freeze upside down until set.
Once chocolate has hardened, remove from freezer and pipe in marshmallow mixture until domes are full. Refrigerate to set.
Place remaining chocolate in the heat proof bowl and place back over the saucepan of simmering water, stirring frequently until chocolate melts. Once melted, remove chocolate from heat, add baking powder and citric acid and stir to combine. Allow to cool slightly until ready to assemble.
To assemble, use a round cutter to cut 10 x 6cm discs from biscuit base and set aside on a cooling rack set over a baking tray.
Remove the Parfait from the freezer and use the same cutter to cut 10 x 6cm discs. Place the discs of parfait on top of biscuits.
Unmould the white chocolate and marshmallow domes and place on top of parfait.
Carefully pour the warm white chocolate mixture over the assembled domes and freeze until ready to serve.
To serve, place frozen domes onto serving plate. Sprinkle with lustre dust and scatter with edible flowers.