Preheat your oven to 300 degrees Fahrenheit. Line a large, rimmed baking sheet with parchment paper or a silicone baking mat to ensure the snack mix does not stick to the pan and for an easier cleanup later.
In a very large mixing bowl, combine the rice cereal squares, corn cereal squares, and the pecan halves. Gently toss them together with your hands or a large spatula, being careful not to break the delicate cereal pieces.
In a small saucepan over medium-low heat, melt the butter. Once melted, whisk in the light brown sugar, ground cinnamon, and salt. Continue to whisk until the sugar has mostly dissolved and the mixture is smooth. Remove from the heat and stir in the alcohol-free vanilla extract.
Drizzle the warm butter mixture over the cereal and pecans. Use a large spoon to gently fold the mixture together until every piece of cereal and every pecan is evenly coated with the buttery glaze. Take your time with this step to ensure no dry patches remain.
Spread the mixture in an even layer on the prepared baking sheet. Place it in the oven and bake for 45 minutes. Every 15 minutes, remove the pan and stir the mix gently to ensure even browning and to prevent the sugar from burning on the bottom.
Remove the baking sheet from the oven and let the mix cool completely on the pan. The glaze will harden and become wonderfully crunchy as it cools. Once it is at room temperature, break up any large clumps before serving or storing.
