In a large pot, pop the popcorn according to package instructions. Place the popped popcorn in a large mixing bowl and set aside.
In a medium saucepan over medium heat, melt the butter. Once melted, add the sugar, corn syrup, and salt. Stir continuously until the mixture comes to a boil.
Allow the mixture to boil for 2 minutes without stirring. Remove from heat and stir in the vanilla extract.
Pour the melted mixture over the popcorn and quickly stir to coat all the popcorn evenly. Add the mini marshmallows and mix until they are evenly distributed.
Let the mixture cool slightly, then add the red and blue M&Ms, folding them gently into the popcorn mixture.
Lightly grease a 9x13 inch baking dish or line it with parchment paper. Press the popcorn mixture into the dish firmly to create an even layer.
If using, melt the white chocolate chips in a microwave-safe bowl in 30-second intervals, stirring in between, until smooth. Drizzle the melted chocolate over the top of the popcorn cake.
Allow the cake to cool completely at room temperature for about 1 hour. Once set, cut into squares and serve.