Heat oven to 180°C (170°C for dark or nonstick pans). Grease a 23cm round cake pan.
Make and bake cake as directed on box for 23cm round cake pans. Cool in pan 10 minutes. Remove from pan to cooling racks. Cool completely, about 30 minutes.
In 1-liter saucepan, heat chocolate chips and butter over low heat, stirring constantly, until chocolate chips are melted. Spread over top of cake, allowing some to drizzle down side.
In 1-liter saucepan, melt white chocolate and butter over low heat, stirring constantly. Drizzle over top of cake. Make the white chocolate curls by pulling a vegetable peeler toward you across a white chocolate bar. Decorate cake with curls.