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

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

Pour the melted chocolate onto the prepared baking sheet. Use an offset spatula or the back of a spoon to spread it into an even layer about 1/4 inch thick, leaving a 1-inch border on all sides.

Immediately sprinkle the chopped pistachios evenly over the melted chocolate, pressing them gently into the surface so they adhere.

Sprinkle the flaky sea salt evenly over the top, adjusting the amount to your taste preference.

Let the bark set at room temperature for 1-2 hours until completely firm, or refrigerate for 30 minutes if you're in a hurry.

Once fully set, break the bark into irregular pieces by hand or cut into clean squares with a sharp knife. Serve immediately or store in an airtight container.