For the marshmallows: pour 1/2 of the water into the bowl of a stand mixer and sprinkle the gelatin over the top. Set aside.
Add the honey and remaining water to a saucepan over a medium heat. Bring to a boil, then reduce heat and simmer until it reaches 115C.
When your honey syrup reaches 112C, fit the mixer with the whisk attachment, then turn it on low and beat the gelatin mixture. With the mixer running, carefully pour in honey syrup in a slow, continuous stream. Add the vanilla extract, turn mixer to high, and beat for 10 minutes, until white, glossy peaks form and the mixing bowl feels cool to the touch. Keep the mixer running on low to keep mixture from setting.
For the jam: add the raspberries, icing sugar and lemon juice to a saucepan set over a medium heat. Cook until the raspberries have released their juice and the mixture has thickened slightly. Remove from the heat and set aside.
Heat oven to 180C. Butter, then line, a swiss roll tin, approx 22cm x 30cm. Melt the butter in a saucepan, cool for 5 minutes, then add half the white chocolate and set aside for a couple of minutes to melt. Stir until smooth.
Whisk the sugar and eggs together for about 2 minutes until thick and pale, then add the flour, salt, cooled chocolatey butter and vanilla, mixing until combined.
Pour the mixture into the tin, then add dots of the raspberry jam on top. Take a cocktail stick and swirl through the raspberry jam, forming a swirled pattern on top of the mixture. Dot the remaining fresh raspberries on top of the jam and finally scatter over the remaining half of white chocolate. Bake for 40-45 minutes until just set.
Cool completely in the tin, then top with the marshmallow mixture and gently toast with a torch until golden. Cut and serve. Bars are best eaten the same day.