Put all the ingredients for the sponge in a blender and mix until you have a uniform, creamy batter.
Divide the batter into four equal portions. Add food coloring to three portions: one drop of blue, three drops of purple, and one drop of pink. Put alternating spoonfuls of the different colors into a small springform pan (4 inches) until half full. Stir to create a marbling effect. Bake for 35 minutes at 350°F.
Beat icing sugar, egg white, and salt with an electric whisk until the mixture is silky. Add the butter and beat it in.
Remove the cooled cakes from the pans and cut each one horizontally into three slices. Spread buttercream on each layer and reassemble the cakes. Cover the outside with buttercream.
Divide the remaining buttercream into three portions and color them purple, blue, and white. Spread layers of each color on plastic wrap, roll it up, and place it in a piping bag with a star-shaped nozzle. Dot the cakes with the icing to create hydrangea flowers.