Line a 9x13 inch baking dish with parchment paper, leaving some overhang for easy removal later.
If you haven't done so already, pop your popcorn. You can use an air popper, a stovetop method, or microwave popcorn. Make sure to remove any unpopped kernels.
In a medium saucepan over low heat, melt the salted butter. Once melted, add the marshmallows and stir continuously until completely melted and smooth.
In a large mixing bowl, combine the freshly popped popcorn, melted marshmallow mixture, and 1 cup of M&Ms. Gently mix until the popcorn is evenly coated.
Fold in the sour gummy candies into the popcorn mixture for a burst of flavor and color.
Transfer the popcorn mixture into the prepared baking dish. Using a spatula, press the mixture down firmly to ensure it holds together.
Allow the Popcorn Bars to cool completely at room temperature. Once cooled, lift them out of the baking dish using the parchment paper overhang and cut them into squares or rectangles.
