Preheat the oven to 375 degrees.
Line a 9x13 pan with parchment paper, with some paper hanging over the ends.
Cream together the butter and sugars, just until they're combined.
Add the vanilla and eggs, and beat until just combined.
Sift together the flour, baking soda, and salt and add it to the butter mixture. Beat until just combined.
Stir in the chocolate chips.
Bake the cookies on the middle rack until the middle looks set and no longer jiggly, about 23 minutes.
Let the cookies cool on a wire rack. Remove the cookies by lifting them out by the parchment paper.
To make the ganache: In a three quart saucepan, heat the cream and corn syrup until hot, but not simmering. Remove from the heat and stir in the chips. Stir until they're completely melted and smooth.
Spread the ganache all over the cookies and let set about five minutes. Decorate with M&Ms.