Line a 9-inch square baking pan with parchment paper, leaving a 2-inch overhang on all sides. Working in 2 batches, process pistachios in a food processor until a very fine powder forms, about 40 seconds. Transfer to a medium bowl.

Combine sugar and ⅓ cup water in a medium saucepan over medium, stirring constantly to dissolve sugar. Continue cooking, undisturbed, until syrup reaches one-string consistency, about 230°F on a candy thermometer, 4 to 5 minutes. To check for one-string consistency: Remove pan from heat, and dip a spoon into the syrup; let it cool for a few seconds, letting excess drip off, and run your pointer finger across spoon. Rub syrup between your thumb and pointer finger, and gently pull fingers apart. If a single small string is formed and does not break when your thumb and pointer finger are pulled apart, the syrup is ready.

Add pistachio powder, cardamom, and salt to sugar syrup. Reduce heat to low, and cook, stirring constantly, until mixture thickens and resembles wet sand, about 2 minutes. Transfer pistachio mixture to prepared pan; roughly press mixture into pan using a moistened rubber spatula. Lightly grease a small offset spatula with ghee, and press mixture into a flat, even layer. Let stand 15 minutes.

Place white chocolate in a small microwavable bowl. Microwave chocolate on medium (50% power) in 10-second intervals, stirring thoroughly between intervals, until melted and smooth, 2 to 3 minutes total. Pour melted chocolate over pistachio mixture in pan, and gently tilt to cover in an even layer of chocolate. Tap pan a few times on counter to level chocolate. Top with flowers and rose petals. Gently tap pan 2 times on counter to settle toppings. Freeze until set, about 20 minutes. Decorate with additional flowers and gold leaf, if desired. Using parchment as handles, transfer bark to a cutting board; remove parchment. Using a sharp chef's knife, cut bark into angular, bite-size pieces.