In a large mixing bowl, beat the softened butter until creamy. Add both sugars gradually and beat until smooth, scraping sides of the bowl. Add eggs, one by one, beating just until blended, then beat in vanilla.
Mix together the flour, cocoa powder, cornstarch, salt and baking soda. Add flour mixture to the sugar mixture using low speed of mixer or by hand -- don't overbeat. Scrape sides of bowl often.
Dough will be slightly sticky. Scoop out 8 mounds of dough onto a plate lined with plastic wrap. Chill for 30 minutes or until dough is not quite so sticky, then shape into firm balls. Cover the dough balls and chill for at least an hour or until ready to bake.
Put mounds of dough on a heavy duty baking sheet lined with parchment paper. Allow about 5 inches of space for spreading.
Bake at 400 degrees F. for 5 minutes, then reduce heat to 350 and bake for another 7 to 9 minutes or until you start to see cracks in the top of the cookies. They will spread quite a bit. Alternatively, you can bake the cookies at 375 for about 15 minutes. I recommend baking one cookie to determine the best bake time for your own oven.
Let the cookies cool completely on the baking sheet, then transfer to a wire rack when you are ready to make the ganache.
Mix the chopped chocolate, corn syrup and butter together in a small bowl.
Heat 1/3 cup of cream until simmering, then add the chopped chocolate mixture. Stir until creamy and smooth. If it is very thin, let cool slightly and thicken. Otherwise, spoon over the cooled cookies. Sprinkle with colored sprinkles or colored candy coated chocolates.
If you can't find the rainbow chips, try using the candy coated chocolates from a box of Funfetti Brownies.