Combine flour, cornstarch, salt, and cinnamon in a bowl.
In another bowl, cream the butter and turbinado sugar using a hand mixer.
Add the egg and vanilla. Beat until incorporated.
Next, add the flour mixture and beat until it comes together. It may look dry and crumbly at the beginning, but keep beating, it will come together into a smooth batter. If it becomes difficult with a hand mixer, use your hands or а spatula to knead the dough.
To shape each cookie, use a cookie scooper (1 ½ diameter) for even-sized cookies, or use a spoon. Roll each cookie into a ball, then roll it in the granulated sugar (optional) and place it on the parchment paper-lined baking tray, leaving about 2 inches between each one. Then, using a ½ tsp measuring spoon or your thumb, make the indentation where the jam will nest (see photos). Transfer them to the fridge for at least 30 minutes.
Fill each cookie nest with pumpkin jam.
Bake in a preheated oven at 350F (180C) for about 15 minutes, or until the edges are slightly golden.
Remove the tray from the oven and let the cookies cool before transferring them to a cooling rack.