In a large mixing bowl, combine the all-purpose flour, soft unsalted butter, granulated sugar, vanilla sugar, large egg, and unsweetened cocoa powder. Mix until a smooth dough forms. Wrap the dough in plastic wrap and refrigerate for 30 to 60 minutes.

In a small saucepan, heat the heavy cream over medium heat until it starts to simmer. Remove from heat and pour over the chopped white chocolate in a bowl. Stir until melted and smooth. Refrigerate until firm.

Preheat oven to 160°C for fan oven, or 180°C for conventional. Roll out dough to ¼ inch thickness. Cut out circles and create holes in half for a window effect. Bake for 10 to 12 minutes. Cool completely.

Melt dark or milk chocolate, dip window cookies in melted chocolate, and sprinkle with finely chopped chocolate. Let set.

Whip the chilled white chocolate ganache until light and fluffy. Pipe onto each cookie base, fill with cherry jam, and place decorated cookies on top to create sandwich-style treats.