In the bowl of a stand mixer fitted with the paddle attachment, combine softened butter, sugar, vanilla, and beat on medium-high speed until creamed, fluffy and well combined, about 2-3 minutes.
Stop, scrape down the sides of the bowl and add flour and cornstarch. Beat on low speed until just combined, about 1 minute. The dough will now look dry and crumbly. Just scape down the sides and use your hands to combine the dough together.
Scoop out 1 tablespoon amount of dough, roll very well and shape it into a 1 ¼ inch ball. Make sure the ball is smooth with no cracks.
Press the tip of your thumb about a 1/2 inch into the middle of the ball to make an imprint. Repeat until you use up all of the dough.
Place the indented balls on a baking sheet lined with parchment paper. Leaving an inch apart from each other.
Transfer the balls into the fridge and chill for 30 minutes.
Heat the oven to 350°F about 10 minutes before the cookie dough balls are done chilling.
Take the jam out of the bottle, and stir it well. If it’s too thick, you can place it in a microwave safe bowl, and microwave for 10 seconds before using them.
Remove the chilled cookie dough balls from the fridge, and spoon jam into each indented ball. Fill the indent almost full, but not over-flowing.
Bake 10-12 minutes, removing from the oven before they brown.
Remove the cookies to cooling racks using a rigid spatula and allow to cool. Fill the cookies with your favorite jam. Serve and enjoy!