Preheat the oven to 250°F. Line a large baking sheet with parchment paper and set aside.

In a large mixing bowl, combine the Rice Chex cereal, Corn Chex cereal, pretzels, and mixed nuts. Toss gently until evenly mixed.

In a medium saucepan over medium heat, melt the butter. Add the brown sugar and corn syrup. Stir continuously until the mixture is smooth and begins to bubble. Allow it to simmer for 2 to 3 minutes to fully dissolve the sugar.

Remove the saucepan from heat. Stir in the baking soda and vanilla extract. Mix well until the sauce becomes light and foamy.

Gradually pour the toffee sauce over the cereal mixture, stirring gently after each addition to coat evenly. Continue mixing until all pieces are coated.

Spread the coated mixture onto the prepared baking sheet in an even layer. Do not press too tightly to allow air to circulate while baking.

Bake for 45 minutes, stirring every 15 minutes. This ensures even baking and prevents the mixture from sticking or burning.

Remove from the oven and allow the mixture to cool completely on the baking sheet. The coating will harden as it cools.

Once fully cooled, break into clusters and store in an airtight container at room temperature.