Preheat the oven to 180 °C (355 °F). Beat 250 g softened butter with 100 g powdered sugar and 1 sachet of vanilla sugar using an electric mixer on medium speed for 2–3 min, just until smooth and well blended — shortbread should not be aerated, so stop as soon as the mixture looks even.
Sift 190 g flour with 200 g cornstarch over the bowl and fold in on low speed, mixing only until a smooth dough forms. Do not overwork.
Grease a baking tray and dust lightly with flour. Roll tablespoon-sized portions of dough into balls and space them about 3 cm apart on the tray.
Dip a fork into flour, then gently press it onto each ball to flatten slightly and leave a ridged pattern.
Bake on the middle rack for 12–15 min, just until the edges are set and the bottoms are pale gold — they should not brown. Cool on the tray for 5 min, then transfer to a wire rack.