Using an air popper, pop the 1/4 cup of corn kernels. You can also pop the corn kernels in a kettle with some vegetable oil. Keep the kernels moving and the lid on the kettle. I would use 1 tablespoons of vegetable oil. Set popcorn aside.
Spray a large jelly roll pan (17¼ x 11½ Inch) with non-stick cooking spray. This will prevent any sticking.
In a medium saucepan, add the sugar into a dry pan. Place the pan over medium heat. Every minute or two, give the sugar a swirl by picking the pan up and moving it in a circular motion. Don't use utensils. You'll start to see the sugar caramelize. Keep picking up and swirling the pan until all the sugar has melted. It will take about 17 minutes for the sugar to caramelize and melt.
Once the sugar is melted, turn the heat down to medium/low. Add in the salt and vanilla bean paste. Do not use vanilla extract. It will make the caramel gritty.
Add in all of the butter and whisk until the butter incorporates into the caramelized sugar and it becomes thick. It will take a good couple of minutes of whisking vigorously to have the salted caramel come together. I used salted butter, but feel free to use unsalted. Remove from the heat.
Pour the salted caramel into the bowl of popcorn and stir until all the popcorn is coated.
Add in the m&m's and stir to combine.
Spread the popcorn on the prepared jelly roll pan and let cool. As the caramel cools, it will harden.