Heat 2/3 of the dark chocolate in a microwave-safe bowl in 15-second intervals, stirring after each interval, until chocolate is nearly melted but some chunks remain, 1 to 2 minutes. Remove from microwave and stir until completely smooth. Add the remaining 1/3 of dark chocolate. Stir well until all the chocolate has melted.
Melt white chocolate in a microwave-safe bowl in 15-second intervals, stirring after each interval, 1 to 2 minutes.
Lay out a large sheet of parchment paper on a flat work surface. Pour dark chocolate onto parchment and spread into a thin, even layer using a spatula. Drizzle white chocolate on top. Sprinkle cranberries and pumpkin seeds quickly over chocolate before it sets.
Leave the chocolate to set fully, about 45 minutes. Break into randomly sized pieces and store in an airtight container.