In a medium bowl, whisk together the all-purpose flour and salt until well combined. Set aside for later use.

In the bowl of a stand mixer fitted with the paddle attachment, beat the cool room temperature unsalted butter for 1 minute on medium speed. Add 1/2 cup of the granulated sugar and the light brown sugar. Continue mixing until the mixture becomes light and fluffy, about 2 minutes. Scrape down the sides of the bowl as needed. Add the egg yolk and vanilla extract, then mix again until everything is well incorporated. I always find that beating the butter and sugars just a little extra really improves the cookie texture.

Add the prepared dry ingredients from Step 1 to the wet mixture in the stand mixer bowl. Mix on low speed until the dough just comes together and there are no visible streaks of flour. Avoid overmixing to keep the cookies tender.

Scoop out roughly 1 1/2 teaspoons of dough per cookie and roll into balls using your hands. Place the remaining 1/2 cup granulated sugar in a small bowl. Roll each dough ball in the sugar until fully coated. Arrange the sugar-coated dough balls on a parchment- or Silpat-lined baking sheet (you may need two sheets). Use the rounded back of a teaspoon or your thumb to gently press an indentation into the center of each dough ball. Once all cookies have indents, transfer the baking sheets to the freezer and chill for 30 minutes before baking. Chilling helps the cookies retain their shape and prevents spreading.

Preheat the oven to 350°F (175°C). Spoon the raspberry jam into a small zipper-lock bag, then snip a tiny corner from the bag to create a makeshift piping bag. Carefully pipe or spoon about 1/2 teaspoon of jam into each cookie's indentation, making sure not to overfill to prevent spillage during baking. Space the cookies at least 2 inches apart. Bake one sheet at a time for 10 to 12 minutes, or until the edges are just turning light golden brown.

Remove the baking sheet from the oven and let the cookies cool on the sheet. Enjoy your raspberry thumbprint cookies! I like to let the cookies cool completely before transferring them to an airtight container to keep them fresh and prevent the jam centers from smudging.