Line two cookie sheets with parchment paper. Set aside.
In a large saucepan, add the sugar, corn syrup, pumpkin pie spice, and vanilla. Bring to a low boil, stirring frequently.
Carefully add the peanut butter and pinch of salt and stir until the peanut butter is fully incorporated. Turn off the heat.
Add in the Chex cereal and stir well, making sure each piece is coated. Let it sit for 2 minutes.
Pour the marshmallows in and stir until evenly dispersed, but not too long. The marshmallows will start to melt, but you do not want them to melt completely.
Using a 2-tablespoon scoop, scoop out mounds of the Chex mixture onto the cookie sheet. They will not spread much, so you can place them close together.
Let them sit for about 30 minutes to firm up.
Store in layers in an airtight container.