First make the rose petals. Please note they take 24 hours to dry, so start the day before you want to display the cake. In a small ramekin, mix together the egg white and a tsp of water. Line a baking tray with parchment paper. Pick the petals off the rose and wash them in water, then blot dry with a paper towel. Using a small paintbrush that is only used for food, paint the rose petal on both sides with a light coating of egg wash. Sprinkle with granulated sugar on both sides and shake off any excess. Leave to dry on the parchment paper. Repeat for as many rose petals as you wish to use.
Preheat the oven to 338⁰F (170⁰C). Cream together the butter and sugar in a freestanding mixture until fully combined and fluffy. Add the eggs one by one and scrape down the sides of the bowl between each addition. Beat for 1-2 minutes until the mixture is smooth.
In another bowl combine the flour, salt and baking powder, and mix until evenly distributed.
Add the flour to the egg mixture in 3 separate parts. In another jug mix together the milk and the vanilla extract, then add to the mixture, beating on a slow speed. Add the rose water last and mix just until combined.
Line two 8-inch cake pans and spoon the mixture evenly into both. Bake for 30-35 minutes until a skewer comes out clean. Once done allow to cool in the cake pan for at least 45 mins to an hour, then turn out the cake onto a cooling rack and allow to cool completely.
While the cake is cooling, make the buttercream. In the stand mixer, mix the butter on high speed for 5 minutes. Then add the powdered sugar in 3 separate parts and mix on a slow speed until fully incorporated. Add the milk and rose water and beat again on high speed for a further 5 minutes.
Once the cakes have cooled, turn one half onto a serving plate and pipe a ring of the buttercream around the outside. This will keep the jam inside! Warm the jam either in a small saucepan, or in the microwave for 20 seconds to make it just slightly more runny. Then pour on top of the bottom layer of the cake inside the buttercream piping. Place the other cake on top and coat the whole cake with a very thin layer of buttercream. This is called crumb coating and will make frosting the rest of the cake a lot easier. Place the cake in the fridge for about 15 minutes to let the first coat dry slightly.
Once 15 minutes has passed, take the cake out of the fridge and continue to ice the cake. Use as much of the buttercream as you like depending on how thick you like the frosting. Garnish with the rose petals and serve!