Using a large, heavy-bottomed stock pot, melt vegan margarine over medium heat.
While the margarine is melting, use a bit of margarine to grease an 8x8 or 9x9 baking pan and then line with parchment for ease of removal. Set aside.
Once the margarine is melted, add the pumpkin and stir together.
Add the marshmallows, stirring occasionally until melted. Remove from the heat.
Now add the vanilla extract, pumpkin pie spice and salt.
I like to let this mixture cool down a bit before adding the cereal because it tends to make them more chewy & like to retain that "crisp". If it gets too difficult to fold them in, just use the residual heat from the stove to slightly warm it again. The cooler the mixture can be before folding the cereal in, the better the end result.
Using damp hands, press into the prepared baking pan. I used to always butter or spray my hands, but found the mixture still sticks. So I tried damp hands and it worked MUCH better.
Press the mixture gently, but firmly into the pan.
Immediately decorate with jimmies or sprinkles & lightly press them on.
I like to refrigerate for at least an hour to set them before cutting so they retain their shape.
Cut into squares and serve immediately or store in an airtight container, or refrigerated for up to a week (but I doubt they will last that long :)