Put 9 ounces of the chocolate in a microwave-safe bowl. Microwave in 30-second intervals, stirring well between each, until just melted. Stir in the remaining 3 ounces chocolate and let stand 3 minutes. Stir until completely smooth. If there are still some lumps, microwave in 15-second intervals, stirring well after each, to melt any remaining chocolate, but take care not to heat more than necessary.
Pour the chocolate onto the parchment and gently spread to a thickness of 1/4 inch. Sprinkle the pistachios and cranberries on top, pressing very lightly so they adhere. Sprinkle with the flaky salt.
Refrigerate until set, about 20 minutes. Break into pieces and keep refrigerated until ready to serve.