Lightly toast the Chex and popcorn in the oven at 250°F for 5 minutes before mixing with the butter coating to enhance their crunch and prevent sogginess from the sweet syrup.

Fold in the marshmallows, M&Ms, and sprinkles only after the mixture has cooled completely to prevent melting and maintain their vibrant colors and distinct textures.

Melt butter with brown sugar and corn syrup over low heat, stirring constantly for 2-3 minutes until fully combined; this prevents crystallization and ensures an even, glossy coating on every piece.

Spread the coated mixture in a thin, single layer on parchment paper rather than bunching it together, allowing air circulation for uniform crispness and preventing clumping as it cools.

Add baking soda in the last 30 seconds of heating the syrup mixture off the heat—it will foam slightly, aerating the coating for an incredibly light, crispy texture throughout the finished mix.