Preheat oven to 325*F.
Mix sugar, butter, all-purpose flour, and starch to obtain smooth dough. The dough will resemble playing dough.
In the meantime, melt 100 g of semi-sweet chocolate over a water bath, drop in peppermint oil, and stir to combine. Fill the small indentations in the cookie with mint chocolate. Melt the rest of the semi-sweet chocolate and white chocolate and drizzle all over the cookies. Allow the chocolate to harden before indulging.