Line a large baking sheet with parchment paper or a silicone baking mat. Set aside.
In a large microwave safe bowl, toss together the Chex cereal, mini pretzels, Teddy Grahams, and chopped pecans. Set aside.
In a small microwave safe bowl or a glass measuring cup with a spout, melt the butter. Stir in the vanilla extract. Add the brown sugar and the pumpkin pie spice and stir until completely combined. Pour butter mixture over the cereal mixture and stir gently with a spatula until all of the sugar and spice is distributed evenly.
Put the bowl in the microwave on HIGH for 1 minute, then stir gently with a spatula to distribute heat. The Teddy Grahams will start to get slightly soft. This is ok. This is why I encourage you to stir gently. They will get crisp again once Chex mix cools. Heat Chex mix again in 1 minute increments for a total of 3-4 minutes, stirring after each minute. Stop when the Chex mix begins to brown. Pour onto prepared baking sheet and allow to cool. Store Chex mix in an airtight container or large zip-top bag at room temperature up to 2 weeks.