Line a small, rimless baking sheet with foil. Place the dark chocolate in the top of a double boiler set over barely simmering water. (If you don’t have a double boiler, place the chocolate in a metal bowl that fits snuggly over a saucepan of barely simmering water, being certain no steam can escape.) Stir until the chocolate is melted and smooth. Immediately dry the bottom of the double boiler top or bowl, pour the chocolate onto the foil, and spread out a scant ¼-inch thick.
Scatter the walnuts, cherries, and ginger over the chocolate. Sprinkle with the salt.
If desired, melt the white chocolate in the top of a double boiler set over barely simmering water. (If you don’t have a double boiler, place the white chocolate in a metal bowl that fits snuggly over a saucepan of barely simmering water, being certain no steam can escape.) Stir until the white chocolate melts. Using a fork, drizzle the white chocolate over the candy.