:::For the Cake:2 cups (4 sticks) unsalted room-temperature butter, plus more for greasing:::
6 cups cake flour
1 teaspoon salt
2 tablespoons baking powder
1 cup room-temperature whole milk
2 teaspoons vanilla extract
1 teaspoon rose water
4 cups sugar
10 egg whites
1 teaspoon blue food coloring
:::For the Frosting:3 cups unsalted butter, softened:::
6 cups confectioners' sugar
6 tablespoons whole milk
½ teaspoon salt
½ teaspoon vanilla
½ teaspoon rose water
Assorted sprinkles, for garnish
Directions
Make the cake: Preheat the oven to 350° and grease four 8-inch round cake pans with butter, then line each one with parchment.
In a large bowl, whisk together the cake flour, salt and baking powder until incorporated. In a medium bowl, combine the milk, vanilla and rose water. Set both aside.
In a stand mixer fitted with the paddle attachment, beat the butter and sugar on medium speed until light and fluffy, 2 minutes.
Working in 2 batches and alternating, add the flour and milk mixtures to the butter-sugar mixture, mixing to incorporate between each addition. Transfer the batter to a large bowl.
Clean the stand mixer bowl and replace the paddle with the whisk attachment. Add the egg whites and beat them on medium-high speed to firm-peak stage. Carefully fold the egg whites into the cake batter.
Divide three-quarters of the cake batter evenly between 3 of the prepared cake pans. Stir the blue food coloring into the remaining cake batter until smooth and transfer to the remaining prepared cake pan.
Bake until a toothpick inserted into the centers of the cakes comes out clean, 30 minutes. Let cool for 10 minutes, then carefully remove cakes and set them on wire racks to cool completely.
Meanwhile, make the frosting: In a stand mixer fitted with the whisk attachment, beat the butter and confectioners' sugar until light and fluffy, 2 minutes. Add the milk, salt, vanilla and rose water, and beat until smooth.
Spread 1 cup of frosting each onto 2 of the white cakes. Slice the final white cake in half, crosswise, and layer 1 piece on top of the other two frosted cakes. Using a 4-inch round cutter, make an indent in the center of each cake stack. On one of the cake stacks, using a paring knife, cut a cone out of the center so that the point reaches just into the bottom cake. Using a 1-inch cutter, make another indent in the center of the other cake stack. Using a spoon, scrape out the cake in between the 2 indents to form a doughnut-like shape.
Slice the outer crust of the blue cake to remove any golden pieces. Using a 4-inch round cutter, cut out a round in the center, then thinly shave the outer pieces. Begin layering the blue cake in both the cone-shaped and doughnut-shaped indents to fill completely. Invert the doughnut-indented cake stack on top of the other and press gently. Spread the remaining icing over the cake to form a smooth layer, then garnish liberally with sprinkles. Slice, revealing a heart in the center, and serve.