Line a baking sheet with parchment paper or a silicone mat. Set aside.

Place white chocolate in a microwave-safe bowl. Microwave in 30-second intervals, stirring after each, until melted and smooth (about 90 seconds total). Alternatively, melt in a double boiler over barely simmering water, stirring constantly.

Pour melted chocolate onto the prepared baking sheet. Use an offset spatula to spread into an even layer about 1/4-inch thick (roughly 9x12 inches).

Immediately sprinkle pistachios and cranberries evenly over the surface, pressing gently so they adhere. Tap the pan firmly on the counter 3-4 times to eliminate air bubbles.

If using, sprinkle flaky sea salt lightly over the top.

Refrigerate for at least 2 hours until completely firm and set.

Break into irregular shards by hand or cut into squares with a sharp knife. Store in an airtight container in the refrigerator.