1. To sugar coat the flowers, lightly whisk the egg white with 1tsp water. Add the caster sugar to another bowl. Using a paint brush, lightly brush each flower all over with the egg white, then sprinkle with the sugar. Gently shake, then place on a wire rack to dry for at least 4 hrs.
2. Heat the oven to 180C/160C fan/gas 4. Grease and line a 33cm x 23cm Swiss roll tray with baking parchment. Using an electric whisk, mix the eggs, sugar and Dr. Oetker Vanilla Extract in a large bowl for 5 mins until light and creamy (the mixture should leave a trail when you remove the whisk). Fold in the flour and Dr. Oetker Baking Powder, then spread the mixture into the prepared tray. Bake for 10-12 mins, until cooked through (the sponge should spring back when pressed). Leave to cool in the tin for 10 mins.
3. Place a sheet of baking parchment (slightly bigger than the tray) onto your work surface and dust with a little caster sugar. Turn the cooled sponge out onto the parchment, peel off the baking parchment backing, then cover with a clean tea towel and set aside to cool completely.
4. Meanwhile, melt the 100g Dr. Oetker White Chocolate in a heatproof bowl set over simmering water. Stir until melted, then pour half onto a lined baking tray and spread out to an even thickness. Divide the remaining chocolate between two small bowls. Mix in 1-2 drops of Dr. Oetker Green Food Colour Gel in one and 1-2 drops of Dr. Oetker Yellow Food Colour Gel in the other. Spot each of the coloured chocolates onto the white chocolate in the tray, then gently swirl through with a cocktail stick to create a marbled effect. Leave to set, then cut into shards using a sharp knife.
5. In a bowl, sift the icing sugar over the mascarpone and Dr. Oetker Vanilla Bean Paste, then whisk until smooth. Using a stand mixer with a whisk attachment, whip the double cream until it holds firm peaks, then gently fold into the mascarpone. Transfer a quarter of the mixture into a piping bag fitted with a large star nozzle. Set aside until ready to decorate.
6. Spread the remaining cream mixture all over the cool sponge, leaving a small margin all around the edges (so it doesn’t ooze out when rolled up), then top with the raspberry halves. With the shortest side of the sponge facing you, lightly score a line about 2cm in from edge, then roll tightly, using the paper to help you. Make sure to place the join on the bottom, then sit on a serving plate. Pipe the remaining cream down the centre on top of the roulade, top with a few whole raspberries, the white chocolate shards and the sugar-coated flowers. Best served within 2 hrs.