Preheat the oven to 350 degrees F. Line a 9x13 inch pan with nonstick foil or parchment paper.
In the bowl of a stand mixer with a paddle attached, beat the butter, salt (if using), both sugars, cocoa powder, corn syrup and vanilla until well mixed, scraping sides of bowl often.
With mixer on lowest speed, add the eggs one at a time, stirring just until mixed. Add the oil and stir until incorporated, then stir in the flour, scraping bowl. This takes some elbow grease because there's a lot of batter and it tends to stick to the bottom of the bowl.
Spread the batter in the pan and bake at 350 degrees F. for 45 to 50 minutes or until brownies appear set.
Sprinkle some M&Ms and chocolate chips over the hot brownies. They should melt a little and adhere. If not, you can drizzle melted chocolate over the brownies later and THEN add the toppings.
Let cool at room temperature, then chill for at least 3 hours before slicing. I'm convinced brownies taste better the second day, but that's just me.
Cut into 16 large or 32 small brownies. Store in refrigerator.