These buttery shortbread thumbprint cookies filled with raspberry jam are so delicious and festive for holiday parties as a special dessert.
Line two large rimmed baking sheets with parchment paper or silicone baking sheets, and refrigerate.
In a medium-sized mixing bowl, use a spatula to combine flour, cornstarch, and salt, and set aside.
Divide the cookie dough into approximately quarters. Shape one of the quarters into a log and divide it into 7-8 pieces, approximately 1 tablespoon each. Roll each piece into a ball and place evenly on a prepared baking sheet. Repeat until all the dough is rolled.
Use a rounded 1-teaspoon measuring spoon to make an indentation into the dough. Dip the spoon into a small bowl of flour to prevent sticking as needed. Use your fingers to pinch together any cracks, the dough is very pliable. Repeat until all the dough balls are shaped with indentations. Refrigerate the baking sheets with prepared cookie dough for at least 2 hours.
When ready to bake, check that the oven rack is in the middle, and preheat the oven to 350°F. While waiting for the oven to preheat, fill the indentations with a heaping 1/2 teaspoon of raspberry jam. Return the cookie trays to the refrigerator until the oven is ready.
Bake each tray separately for 15 minutes until golden brown. (If your oven bakes evenly, bake both trays at the same time but rotate them halfway.)
Allow the cookies to cool on the tray for at least 20 minutes before eating or handling them.