Fill half-sphere molds with some mousse. Spread remaining mousse up the sides of the round mold.
Unmold frozen layers; press into mousse so edges are filled. Freeze overnight.
Pipe three white chocolate rings on acetate (one wide, two narrow); chill to set.
Spray frozen cake with white velvet spray outdoors.
Gently warm glaze to 40 °C, stir, cool to 34 °C, and pour over inverted cake set on an upturned bowl. Smooth edges.
Glaze half-spheres on a rack. Quickly place them atop, add chocolate rings, and finish with edible flowers.