Preheat oven to 350 degrees F (180c). Line two baking sheets with parchment paper or silicone baking mats.
Sift the flour, salt and cornstarch into a large bowl. Repeat process for the confectioners sugar into a separate bowl. Set aside.
2 cups all purpose flour (270g), 2 tablespoons cornstarch (20g), ¼ teaspoon salt, 1 cup confectioners sugar (100g)
In a large bowl, cream the butter using a hand mixer for about 2 minutes. If using a stand mixer, for about 4 minutes. Note: Using a hand mixer, mixes faster than a stand mixer.
3 sticks unsalted butter (270g)
Once the butter is soft and fluffy, add the confectioners sugar. Mix with the whisk first (without the mixer being on) to prevent from flying out of the bowl.
Mix on speed #2 to integrate. Scrape down sides of the bowl with a spatula and continue mixing for about 1 minute.
Add the vanilla and egg. Beat for about a minute until everything is integrated and fluffy. Clean the mixer whisks and add into the bowl.
1 medium egg at room temperature, 1 teaspoon vanilla extract
Add half of the flour mixture and integrate with the spatula (using a spatula to mix helps the butter to stay airy at this point). Add the rest of the flour and fold to integrate all ingredients.
Transfer the dough to a decorating bag fitted with a large open star tip. Pipe the dough into 2-inch circles on the parchment paper or silicone baking mats.
For cookies with jam in the middle: Use the tip of your finger to gently press an indentation in the center each cookie dough ball. Add a little bit of your favorite jam, cajeta, nutella or dulce de leche.
Bake, one pan at a time, until light golden brown at the bottom, 12 to 15 minutes. Allow to cool completely before storing or serving.