Heat your heavy cream in the microwave for 15 to 20 seconds, until very hot but not boiling. Place your white chocolate in a small bowl and pour the hot cream over the top. Let sit for 4 minutes, then whisk until completely smooth. If ganache is too thick to drip nicely, pop it in the microwave for a few seconds.**
Transfer to a squeeze bottle (or a large zip-top plastic bag with one corner snipped off) and squeeze a line of white chocolate around the perimeter of the chilled cake, squeezing harder in some places to create drips down the side of the cake. Use the remaining ganache to fill in the top of the cake. Refrigerate until ganache is set, then make the dark chocolate drip.
Transfer the ganache to a plastic squeeze bottle (or a large zip-top plastic bag with one corner snipped off). Squeeze a circle of ganache around the edge of the cake on top of the white chocolate, applying a little extra pressure at certain points to create drips. Carefully spread the remaining ganache over the top of the cake. If desired, decorate the top of the cake with chocolate candies. Refrigerate to set the ganache, then slice and serve.
*Don’t use white chocolate chips
**When making your ganache drip, if the ganache is too thin after you stir in the cream, just let it sit at room temperature until it thickens sufficiently.