To make the pastry dough, cut the butter into the flour, powdered sugar, and vanilla sugar with a pastry knife. Alternately, pulse it together in a food processor until it resembles coarse crumbs.

Add the egg (and vanilla extract, if using instead of vanilla sugar) and mix to bring the dough together. (Or pulse a few more times in the food processor.) Do not overwork the dough.

Divide the dough into two even portions, wrap and refrigerate for at least an hour (or up to 2 days).

Once chilled, preheat the oven to 350 degrees (175 C or Gas Mark 4).

Remove the dough from the refrigerator and roll it out on sheets of parchment paper. Each sheet should be roughly 12x16 inches (30x40 cm).

Transfer each sheet to a baking tray and prick with a fork.

Bake each sheet for 10-15 minutes, until just barely browned but cooked through.

Allow the sheets to cool completely on their baking trays before proceeding with assembly.

To assemble, spread one layer of shortcrust pastry with raspberry jam.

Gently flip the second layer onto the first, creating a sandwich of shortcrust pastry with jam in the center.

Prepare white icing by mixing roughly 2 cups of powdered sugar (250 g) with about 2 tablespoons of water. Icing should be a bit stiff so it doesn't run, but add a bit more water if necessary to get a smooth consistency (work quickly, the icing sets fast and cannot be prepared ahead).

Spread the icing over the top layer of shortcrust pastry.

Decorate with sprinkles, crushed freeze-dried raspberries, or leave plain.

Slice off the edges of the cookie sheets to form an even rectangle with clean corners.

Slice the individual bars by cutting the sheet into 2 rows of 6 bars, for a total of 12 hindbærsnitter.

Enjoy immediately, or store in a tightly sealed container at room temperature for up to 2 weeks, or freeze for longer storage.