Preheat oven to 325°F and Line a baking sheet with parchment paper and set aside.
In a large mixing bowl or bowl of a stand mixer fitted with paddle attachment beat butter and both sugars until light and fluffy. Make sure to take the time during this step as this will determine how your cookie dough turns out. If beaten well, in the end, the texture should melt in your mouth immediately. If beaten too little, the cookies may taste similar but not as good as they should. The batter should be somewhat sticky.
Slowly add egg yolk, vanilla, and salt. Beat on low until well combined.
Add the flour to the butter mixture and combine. The batter will be somewhat sticky. If using gluten-free flour blend, depending on the brand the batter may be overly sticky. In this case I recommend refrigerating or freezing the dough until it is easy to manage. The cookies will not change in terms of texture or flavor after freezing. But they will sure spread way less.
Scoop the dough into portions of about 1 ½ tablespoons each and arrange on the baking sheet lined with parchment paper. Roll each portion into a nice round ball shape, dip in granulated sugar (you need about ¼ cup for this batch) and place back on the tray. These cookies don’t spread as much, so there’s no need to leave more than 1 ½ - 2 inches between each.
Press down the center of each ball with the back of a teaspoon measuring spoon,( the handle-end of a wooden spatula, a small bottle cork or your thumb!), making a deep well in the middle. It should be deep enough to fill with 1 teaspoon of jam but should not be too wide. If you make it wide and not deep, cookie jam may spread out on top of the cookie while baking, making it still eatable but not the prettiest looking cookies.
Spoon about 1 teaspoon of raspberry jam (don’t overfill) into each indentation and transfer the tray with cookies onto the middle rack of the oven. Bake for 10-15 minutes.
Remove from the oven and allow to cool on the baking sheet for at least 10 minutes or until firm. Sprinkel more confectioner’s sugar if desired.