Preheat your oven to 180°C (350°F) and line two baking sheets with parchment paper to prevent sticking.
In a large mixing bowl, whisk together the flour, baking soda, salt, and cinnamon until evenly combined. This creates a flavorful dry base for the cookies.
In a separate bowl, beat the softened butter and brown sugar together using an electric mixer or whisk until the mixture is light, fluffy, and creamy, about 2-3 minutes. The sound should be a gentle whir, and the texture airy.
Add the pureed pumpkin to the butter mixture and beat until well incorporated, then mix in the egg until the batter is smooth and slightly thickened.
Gradually fold the dry ingredients into the wet mixture, stirring just until combined. The dough will be thick and sticky, resembling a soft cookie dough.
Scoop tablespoon-sized portions of dough onto the prepared baking sheets, spacing them about 2 inches apart to allow for spreading. Use a cookie scoop for even size and shape.
Bake the cookies in the preheated oven for 12-15 minutes, until the edges turn golden and the centers look set but still soft. You'll hear a slight crackle when they're ready.
Remove the baking sheets from the oven and let the cookies cool on a wire rack for at least 15 minutes. This helps them set and makes handling easier.
Once cooled, gently spread a generous layer of pumpkin-flavored filling between pairs of cookies to assemble the whoopie pies.
Press the cookies together gently to create a sandwich, ensuring the filling reaches the edges for a perfect look.
Serve the pumpkin whoopie pies at room temperature, enjoying their soft texture and warm spices with every bite.
