Preheat the oven to 180°C. Line two baking trays with baking paper. Combine the ground pistachio, flour, egg whites, vanilla and almond essence, lemon zest and icing sugar in a large bowl, stirring until a thick paste forms. Roll 1 tablespoon of dough into a ball, then place on trays and flatten slightly. Repeat with remaining dough, leaving a 4cm gap between each. Bake for 10 minutes or until cooked through. Dip one half of each biscuit in melted chocolate, allowing excess to drip off, and place on a baking paper-lined baking tray. Sprinkle slivered pistachios and rose petals over the chocolate. Repeat with remaining biscuits, then chill for 15 minutes to firm up before serving.