Place an oven rack in the center position and preheat oven to 350°F. Flip a 12-cup muffin tin upside down so the bottoms of the cups face up.
Melt the butter in 20-second bursts, pour into a shallow bowl, and let cool slightly. Stir together the granulated sugar and cinnamon in a second shallow bowl.
Stack two or three tortillas on a cutting board. Press the 4-inch round cutter into the tortillas and cut out three circles from each large tortilla, aiming for 18 circles total.
Gently poke each circle four or five times with a fork. Brush both sides of one tortilla circle with melted butter, covering it from edge to edge.
Press the buttered tortilla into the cinnamon sugar, then flip it and press the other side. Coat generously and repeat with the remaining circles, stirring the cinnamon sugar now and then.
Drape each coated tortilla over the space between two cups on the upside-down muffin tin so it folds into a taco shape. Fit 10 to 12 shells on the tin at a time.
Bake for 12 to 14 minutes, until the shells are light golden brown and the sugar looks bubbly. Watch closely in the last two minutes. Repeat with remaining tortillas.
Let the shells rest on the muffin tin for 5 minutes, then transfer to a wire cooling rack. Cool for at least 20 minutes, since the shells crisp up as they cool.
Beat the softened cream cheese with a hand mixer on medium speed for 1 to 2 minutes, until smooth and fluffy. Scrape down the sides of the bowl.
Add the pumpkin puree, powdered sugar, vanilla extract, pumpkin pie spice, and salt. Beat on low until combined, then on medium for another minute until silky and smooth.
Spoon the filling into a piping bag fitted with a large round tip, and refrigerate it while you whip the cream.
Pour cold heavy whipping cream into your chilled mixing bowl. Add the powdered sugar and vanilla extract.
Beat on medium-high speed for 2 to 3 minutes, until stiff peaks form and the cream holds its shape. Stop as soon as you reach stiff peaks.
Transfer the whipped cream to a second piping bag fitted with a large star tip. Line up the cooled shells on a serving platter so they lean against each other.
Pipe a generous swirl of pumpkin filling into each shell, filling it about two thirds of the way. Top the pumpkin layer with a swirl of whipped cream.
Dust the whipped cream lightly with ground cinnamon and sprinkle on the chopped pecans if using. Serve right away while the shells are at their crispiest.