In a large bowl, add the room temperature butter and sugar. Beat at medium speed until smooth and creamy, scraping the sides as needed. Add in the almond, vanilla extract, cornstarch, and salt and beat until combined. Add flour into the bowl. Beat together at low speed, scraping bowl as needed, until ingredients are all combined. Do not overmix.
Shape the cookies into small balls -- the size of 1 tablespoon (I use a tablespoon measuring spoon to form and measure balls) and then if desired, roll the cookie dough balls in the 1/2 cup granulated sugar until coated. Next, gently Indent the center of each cookie dough ball with your finger. Place these cookies 2 inches apart on a silpat or parchment paper lined cooking sheet. Cover and place in the fridge for 1 hour (or freeze for 30 minutes). Remove from the fridge and add jam to fill (but not over fill) the centers of all the cookies. Return to the fridge for 10 minutes.
Preheat the oven to 350 degrees F. Bake the cookies for 8-10 minutes or until very lightly browned (slightly underbake for soft and delicious shortbread cookies!)
Remove cookies to a cooling rack and allow them to cool completely. Meanwhile, prepare the glaze by whisking all the ingredients together in a bowl until smooth. (Add heavy whipping cream or milk until you've reached your desired consistency; I like the glaze on the thicker side).
Drizzle the glaze over the cookies and allow to set. Cookies best enjoyed within 3-4 days; store in an airtight container.