In a large mixing bowl, combine softened butter, powdered sugar, vanilla sugar, and salt. Beat until smooth and creamy, about 2-3 minutes. Add eggs one at a time, mixing thoroughly after each addition. Gradually fold in all-purpose flour and baking powder until just combined, then knead until smooth. Divide the dough into three equal parts.

For Pistachio-Lemon Cookies: Mix ground pistachios and lemon zest into one portion of the dough. Roll out to about ½ cm thick, cut into shapes, and refrigerate for 15-20 minutes.

For Chocolate Sticks: Mix cocoa powder into the second portion of the dough, shape into 20g balls, and roll into sticks. Chill for another 15 minutes.

For Peanut Thumbprint Cookies: Shape the last portion into 18g balls, coat in egg white and chopped peanuts, make a thumbprint, and refrigerate for 15 minutes.

Prepare the jam filling by heating strawberry jam and granulated sugar in a saucepan until bubbling, then cool completely.

Preheat the oven to 190 °C (375 °F). Bake all three types of cookies for 10-15 minutes or until lightly golden. Cool on baking sheets before transferring to wire racks.

Melt white chocolate with sunflower oil and dip half of each Pistachio-Lemon Cookie, then sprinkle with chopped pistachios. Dip Chocolate Sticks in milk chocolate and sprinkle with chopped hazelnuts.