In a medium bowl, whisk together the cornstarch, all-purpose flour, baking powder, baking soda, and pumpkin pie spice. Ensuring the cornstarch is well-distributed is key to the cookie’s signature texture.
In a large mixing bowl, beat the softened butter and granulated sugar together until the mixture is light and fluffy. This should take about 2 to 3 minutes using an electric mixer.
Add the egg yolks, pumpkin puree, and vanilla extract to the butter mixture. Beat well until fully combined and the color is uniform.
Gradually add the dry ingredients to the wet ingredients, mixing on low speed until a soft dough forms. Do not overmix; stop as soon as the flour disappears.
Shape the dough into a flat disk, wrap it tightly in plastic wrap, and refrigerate for at least 1 hour. Chilling makes the dough easier to handle and prevents the cookies from spreading too much.
Preheat your oven to 350°F (175°C). On a lightly floured surface, roll the dough out to about 1/4 inch thickness. Use a round cookie cutter to cut out circles and place them on a parchment-lined baking sheet.
Bake for 10 to 12 minutes. The cookies should be set but still pale; do not let the edges brown. Remove from the oven and let them cool completely on a wire rack.
Spread a generous teaspoon of dulce de leche on the flat side of one cookie. Top with a second cookie to create a sandwich. Gently press down so the filling reaches the edges.
Dust the finished sandwiches with a light coating of powdered sugar before serving.