Preheat the oven to 350 F. Grease and flour three 6-inch round cake pans that are 3 inches deep.
In a medium bowl, mix together the flour, baking powder, and salt together. Set aside.
In a separate bowl, beat together the egg whites, rose, vanilla, and oil together. Set aside.
In a stand mixer, beat the granulated sugar and butter together until light and fluffy. Slowly begin to alternate adding the flour mixture (in three additions) and the egg mixture (in two additions). Begin and end with the flour mixture. Mix until just combined.
Evenly divide the batter among your cake pans. Bake in the middle rack for about 25 – 28 minutes, or until a toothpick inserted in the middle of the cakes come out clean.
Allow the cakes to cool in the pans for 10 minutes. Run a knife between the edges of the cake and the pan. Turn out the cake onto a wire rack and allow to cool completely. Once cool, wrap tightly with plastic wrap and freeze until needed or for at least two hours. Freezing makes it easier to level the cake and frost.
Make the rose syrup. Combine the ingredients to a small saucepan and heat until the sugar dissolves and begins to boil. Allow to boil for one minute, then remove from heat. Allow to cool to room temperature before transferring to an airtight container to chill in the refrigerator.
Make the frosting. In a clean and dry heat-proof stand mixer, add the egg whites and sugar together. Pour a small amount of water in a small saucepan and boil the water. Place the mixing bowl over the water and beat the egg white mixture until the sugar is dissolved and the mixture is whitened and reaches 165 F.
Remove from the heat and place on the mixer. Beat until it reaches stiff peaks. Add the butter one tablespoon at a time until it is all added the buttercream is silky smooth. Please read notes for troubleshooting the buttercream. Add in the rose syrup and the vanilla and mix in well.
Level the cakes by cutting off the domed tops. Brush the tops of the cakes with the rose syrup
Add a generous amount of frosting on top of two of the cake layers and add raspberries to these two layers as well. Layer the cakes on top of one another, leaving the unfrosted cake for the top layer. Once assembled, add frosting to the top layer and the sides of the entire cake. Smooth out the frosting and chill in the refrigerator as you make the drip.
Make the drip by adding the raspberries and powdered sugar together, mashing and mixing as much as possible. The mixture should be lumpy. Slowly add the milk one teaspoon at a time, mixing well between each addition. The mixture should be thick and paste-like with only a little movement.
Remove the cake from the refrigerator and add the drizzle on top of the cake, pushing it to the edges and allowing it to drizzle down naturally on its own.
Finish with an optional rose on top of your cake.
Notes
If your buttercream seems soupy, simply place the buttercream in the refrigerator for about 20 minutes, then beat again until it comes together. If the buttercream seems too lumpy and the buttercream has flecks of unincorporated butter, allow the frosting to sit out at room temperature for about 15 – 20 minutes, then beat again.