Heat oven to 180°C. Line 2 (45x33cm) half-sheet pans with cooking parchment paper; spray paper with baking spray with flour.
Make both cake mixes as directed on packages, using water, oil and eggs. Pour batter into pans; spread evenly with offset spatula.
Bake 15 minutes or until cake springs back when touched lightly in center. Cool completely in pans on cooling racks.
Using 7cm round cookie cutter, cut each cake into 12 rounds.
For each cake, stack 2 vanilla cake rounds and 2 chocolate cake rounds, alternating flavors and spreading frosting in between layers. Pipe white frosting on cake with chocolate rounds on top, and pipe chocolate frosting on cake with vanilla rounds on top. Decorate with sprinkles.