Beat the Unsalted Butter (1 cup) on high speed with a handheld or stand mixer until creamy, about 1 minute.
Switch mixer to medium speed and add the Granulated Sugar (2/3 cup), Vanilla Extract (1 teaspoon) and Almond Extract (1/2 teaspoon), scraping down the sides and the bottom of the bowl as needed.
Turn the mixer off and add half of the All-Purpose Flour (2 cup) (+ 2 Tbsp); turn the mixer on low and slowly beat just until combined. Add remaining flour and again mix slowly just until a very soft dough is formed. Press the dough down into bowl, tightly cover with plastic wrap to chill overnight.
The next day, remove the cookie dough from the refrigerator an hour before you intend to start baking.
Preheat oven to 350 degrees F (176 degrees C). Line baking sheets with parchment paper (parchment paper seems to work better than silicone mats for these shortbread cookies).
Roll the cookie dough into 1-inch balls, making sure they're nice and smooth. Make an indentation with your thumb into each ball. The dough may crack slightly when you press your thumb into it, but you can pinch the dough back together gently.
Fill each impression with a scant 1/2 tsp of Jam (1/2 cup). Place prepared cookies in the refrigerator for 30 minutes or freeze for 10 minutes – this step is MANDATORY!
Bake the shortbread thumbprint cookies for 12 to 14 minutes, or until very lightly browned on the edges. The cookies will puff up and spread slightly. Do not overbake!
Remove from oven and immediately spoon an additional ½ teaspoon Jam (1/2 cup) into each cookie, smoothing it around to fill the indentation evenly. Allow the cookies to cool on the baking sheet. Cool to room temperature before glazing.
To make the glaze, whisk together the Powdered Confectioners Sugar (1 cup), Cream (2 tablespoon), Vanilla Extract (1 teaspoon) and Almond Extract (1/2 teaspoon) and Light Corn Syrup (1 teaspoon) until smooth. Add more liquid to thin out or add more powdered sugar to thicken to your desired consistency.
Carefully drizzle over cooled cookies and allow glaze to set (you can refrigerate to speed this process up).