Spray a roasting pan with cooking spray, set aside, and pre-heat oven to 300 degrees.
Pop the popcorn using air popper or your other favorite method of popping popcorn.
In a small saucepan, melt butter on medium and add sugar, corn syrup, and salt. Whisk to combine the sugar and bring to a simmer, stir constantly and cook for 1-2 more minutes.
Turn down to low from heat and add vanilla and baking soda and whisk until well combined.
In a small bowl, add mini M&Ms.
Remove popcorn from oven. Separate out 1/2 of the popcorn into a large bowl and set aside. With the remaining half that's in your pan, pour 1/2 of the syrup over the popcorn and mix well.
With gloved hands, gather a palm full of popcorn and firmly press between your palms to form balls. Work quickly because once it all cools too much, you wont be able to form the balls. Set balls on a large plate and complete this with this half of the popcorn.
Gently stir your syrup mixture on stove to ensure that it's not burning.
At this point you can start to add some mini M&Ms to the slightly sticky balls at this point, but don't worry about fully filling them yet.
Add remaning popcorn to the pan and pouring remaining syrup over it and mix well. Form into balls.
Add more mini M&Ms by using some of the syrup at the bottom of your pan as "glue" to stick on more candies to the popcorn balls. Do this before the syrup cools completly a hardens.
Let popcorn balls cool completely and then wrap individually in plastic wrap and store for up to 5 days.