For the crust: Add the yeast to the warm water and let bloom for 10 minutes.
Meanwhile, add the flour and salt to the bowl of a food processor and pulse 5 times to mix. Add the pumpkin puree, olive oil and honey to the yeast mixture and whisk to combine. With the food processor running, slowly add the liquid in a thin stream until a ball forms (you won't necessarily add it all before this happens). Once the ball forms, process for 30 seconds to knead. Turn out onto a lightly floured work surface and knead a couple times to form into a tight ball. Place in a lightly oiled bowl and cover with a clean kitchen towel. Set aside at room temperature until doubled in size, 1 1/2 to 2 hours.
For the white sauce: Combine the ricotta, mozzarella, milk and oregano in a medium bowl. Season with salt and pepper, and mix until well blended.
For the toppings: Preheat the oven to 400 degrees F.
Slice the butternut squash crosswise into 1/2-inch slices. Place in a bowl, drizzle with olive oil, sprinkle with salt and pepper, and toss to coat. Transfer the squash to a baking sheet and roast until slightly browned and softened, 30 to 35 minutes. Slice into triangles.
Raise the oven temperature to 500 degrees F and set a pizza stone on the top rack.
Divide the dough into 2 portions and roll into balls. On a floured work surface, roll out 1 of the dough balls into a 12- to 14-inch circle and place on a well-floured pizza peel. Working quickly, spread with half of the white sauce in a circular pattern, sprinkle with 1 cup of the shredded mozzarella, and arrange some pepperoni if using around the perimeter of the pizza near the crust. Transfer to the pizza stone and bake until the crust is browned, 12 to 15 minutes. Repeat with the remaining dough, white sauce and mozzarella.
When the pizzas come out of the oven, arrange the roasted butternut squash on top to look like a jack-o-lantern: Use 2 triangles for eyes and 2 black olives as eyeballs, then use 1 triangle for the nose and more triangles for teeth. Garnish with basil to make the pumpkin stem.