Place the popped popcorn and the mini pretzels into a large heat-proof mixing bowl. Gently toss them together to ensure they are evenly distributed. Make sure to remove any unpopped kernels from the popcorn so they don’t end up in your final mix.
In a medium saucepan over medium heat, melt the butter and brown sugar together. Stir constantly until the mixture begins to bubble. Let it boil for about 1 to 2 minutes without stirring too much, which allows the sugar to dissolve into a smooth syrup.
Remove the saucepan from the heat. Immediately stir in the vanilla extract, sea salt, and baking soda. The mixture will foam up slightly; this is normal and helps create a light, airy coating for the popcorn. Stir until the glaze is smooth and glossy.
Carefully pour the hot syrup over the popcorn and pretzel mixture. Using a large spatula or spoon, gently toss the mix until every piece is lightly coated with the sweet glaze. Work quickly before the syrup begins to harden.
Spread the coated mixture out onto a baking sheet lined with parchment paper. Let it cool completely at room temperature for about 10 minutes. Once the mix is no longer warm to the touch, sprinkle the M&Ms or chocolate chips over the top and toss one last time.