Preheat oven to 250F. Spray a large baking sheet (or two medium baking sheets) with non-stick cooking spray; set aside.

In a large mixing bowl, combine the Crispix and pecans; set aside.

In a medium saucepan, combine the brown sugar, corn syrup, and butter. Heat over medium heat until the mixture comes to a boil, stirring often.

Remove the sugar mixture from the heat and add the vanilla extract, then stir in the baking soda until fluffy.

Pour over the Crispix and pecans and toss to coat.

Spread the Crispix mix evenly onto the prepared baking sheets. Bake at 250F for 1 hour, stirring every 20 minutes.

Remove the baking sheets from the oven and transfer the Crispix mix onto wax paper or parchment paper to cool to room temperature. If needed, break larger chunks of the snack mix into bite-sized pieces.

Store the snack mix in an airtight food storage container at room temperature for up to 1 week.