Using a spoon or a pastry spatula, mix the two types of sugar with the butter. Add the egg and vanilla sugar, and mix until all ingredients are well combined.

Sift the flour, cornstarch, baking powder, and 1/4 teaspoon fine salt. Add them to the mixture and mix until you get a fully compact dough. Now add the chocolate chips and mix well.

Scoop out dough with a spoon and shape it into round balls. Place them on a tray until all the dough is used. Chill them in the refrigerator for 1 hour or in the freezer for 10 minutes (to prevent them from spreading too much in the oven).

After the chilling time, place about 12 balls (spaced apart) on a baking sheet lined with parchment paper and bake them in a preheated oven for 18 minutes at 350°F. When the edges start to turn golden, it's time to take them out. In total, you should get about 20 cookies. Remove from the oven and let them cool.