Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large mixing bowl, combine the Rice Chex cereal, pretzel twists, and mixed nuts. Set aside.
In a medium saucepan over medium heat, melt the butter. Stir in the brown sugar and corn syrup until completely dissolved.
Bring the mixture to a gentle boil, then let it boil for 4-5 minutes without stirring. Remove from heat and stir in the vanilla extract and baking soda. The mixture will bubble up.
Pour the toffee mixture over the cereal mixture in the large bowl. Gently stir until all the cereal and other ingredients are evenly coated.
Pour the coated mixture onto the prepared baking sheet and spread it out in an even layer.
Bake in the preheated oven for 15-20 minutes, stirring every 5 minutes to ensure even toasting. The mix should be golden brown.
Remove from the oven and let it cool completely on the baking sheet. If using chocolate chips, sprinkle them over the warm toffee Chex mix, and let them melt slightly before mixing.
Once cool, break the mix into pieces and serve in a bowl.