Grease 13- x 9-inch (3 L) baking pan and line with parchment paper. Dust bottom and sides of pan generously with cornstarch, reserving any leftover cornstarch. Set aside.

Pour 1/2 cup (125 mL) cold water into bowl of large stand mixer fitted with whisk attachment; sprinkle gelatin over top. Stir gently; let stand for 5 minutes.

In saucepan set over medium-high heat, heat sugar, corn syrup and 1/2 cup (125 mL) water, stirring to dissolve completely; cook until candy thermometer registers 240°F (115°C), occasionally brushing down sides with wet pastry brush. Remove from heat.

With mixer on low, gently pour hot sugar syrup into gelatin mixture. Slowly increase speed to high; mix for about 10 minutes or until fluffy, stiff and shiny. Add food colouring; mix until marshmallow mixture is uniform orange colour, adding a few more drops if necessary to achieve desired shade.

Stir pie filling with pumpkin pie spice; fold into marshmallow mixture until well incorporated. Pour into prepared pan, spreading to edges and smoothing top. Let stand for 8 hours at room temperature.

Dust top with some of the remaining cornstarch. Flip pan upside down onto parchment paper–lined work surface. Carefully peel back parchment paper; dust top with remaining cornstarch mixture. Cut into squares. Store in airtight container for up to 3 days. (Note: Do not refrigerate.)