1-Finely grind the pistachios in a food: Finely grind the pistachios in a food processor.
2-In a bowl, mix the softened butter,: In a bowl, mix the softened butter, powdered sugar, vanilla sugar, and salt until smooth.
3-Add the egg and ground pistachios, mix: Add the egg and ground pistachios, mix well. Add the flour and baking powder, and knead until smooth.
4-Lightly flour your work surface and roll: Lightly flour your work surface and roll out the cookie dough to just under 1 cm thick.
5-Use a cookie cutter to cut out: Use a cookie cutter to cut out your desired shapes.
6-Place the cookies on a baking sheet: Place the cookies on a baking sheet lined with parchment paper. Bake in a preheated oven at 190 °C (375 °F) for 10-12 minutes, or until lightly golden brown. Let them cool completely.
7-Spread a layer of pistachio paste on: Spread a layer of pistachio paste on one cookie and place another cookie on top. Repeat with the remaining cookies.
8-Melt the white chocolate using a double: Melt the white chocolate using a double boiler: fill a saucepan with 2 cm of water and bring to a boil. Place a heatproof bowl over the saucepan (without touching the water). Add the white chocolate pieces and let them slowly melt, stirring occasionally until smooth.
9-Dip each cookie sandwich into the melted: Dip each cookie sandwich into the melted white chocolate and place them on parchment paper.
10-Decorate with thin lines of white chocolate,: Decorate with thin lines of white chocolate, chopped pistachios, and edible rose petals. Let the chocolate fully harden.