Preheat oven to 350°F. Line two baking sheets with parchment and set aside.
Place dark chocolate into mixing bowl and chop 3-6 sec/speed 6. Transfer into a bowl and set aside.
Place butter, sugar, brown sugar, egg and vanilla sugar into mixing bowl and mix 1 min 30 sec/speed 3. Scrape down sides of mixing bowl with spatula and mix further 30 sec/speed 3.
Add flour, baking soda and salt and knead Dough /20 sec.
Add reserved chopped chocolate and knead Dough /20 sec. With two teaspoons, form dough into walnut-size balls and place them 2 inches apart on prepared baking sheets. Bake one sheet after another for 10-11 minutes (350°F) until cookies are golden brown. Allow to cool on a wire rack before serving or storing in an airtight container.