¾ oz. cornstarch
8 oz. flour
4 oz. butter, room temperature
3½ oz. sugar
2 oz. corn syrup
¼ tsp. baking soda
¼. tsp. kosher salt
2 tsp. pumpkin pie spice, divided
¼ oz. powdered milk
2 egg whites
3 Tbsp. heavy cream
2 Tbsp. brown sugar
½ cup canned pumpkin puree
8 oz. white chocolate, chopped
Preheat the oven 300 degrees. Line 2 baking sheets with parchment paper.
In a mixing bowl, whisk together the cornstarch and flour together. Set aside.
In another large mixing bowl, beat the butter, sugar, corn syrup, baking soda, salt, ½ teaspoon pumpkin pie spice together with an electric mixer on medium-high speed until creamed, about 5 minutes. Add the egg and beat well.
Add 1 egg white and half the flour-cornstarch mixture, beating on low until combined. Add the remaining egg white and the rest of the flour-cornstarch mixture until combined. Pour the batter into a pastry bag with a round tip attached.
Pipe the batter onto the lined baking sheets, creating strips about 2 inches long and ½- inch wide, gently pressing the batter down a bit with the tip while piping so the cookies do not become too thick.
Bake 20 minutes, rotating the baking sheets halfway through, until the edges of the cookies are just slightly brown. Let cool completely on the baking sheets.
Meanwhile, heat the heavy cream, brown sugar and pumpkin puree in a medium saucepan over low heat for a few minutes, stirring, until the mixture combines and just comes to a simmer. Add the white chocolate and let sit for 1 minute, then whisk to combine until smooth. Transfer the melted ganache to a bowl and let cool at room temperature 30 minutes, then cover in plastic wrap and refrigerate until completely chilled.
To assemble the cookies, whip the ganache with an electric mixer on medium until light and fluffy, 1 to 2 minutes. Smear a dollop of ganache on the bottom of a cookie, then press the bottom of another cookie on the ganache to sandwich them together. Repeat until all the cookies are used. The sandwich cookies will keep, refrigerated, for several days.