Preheat the oven to 350℉ and line a baking sheet with parchment paper. Set aside.
In the bowl of a stand mixer fitted with the paddle attachment (or use an electric hand mixer and large bowl), cream the butter and sugar on medium-high speed for 2 minutes, until light and fluffy. Add the egg yolk, then mix again for 1 minute.
Turn off the mixer and scrape down the insides of the bowl. With the mixer on low speed gradually incorporate the flour.
Optional, but recommended: Chill the dough for an hour to prevent the cookies from spreading.
Using a small cookie scoop or spoon, portion and arrange ½ tablespoon-sized dough balls about 2 inches apart on the prepared baking sheet.
Make an indent in the center of the dough with your thumb OR grease the back of a ½ teaspoon measuring spoon with cooking spray and use that to make your indents.
Fill each indent with jam. *I recommend sealing the jam inside of a zip top plastic bag, cutting a very tiny hole in the corner and piping the jam into the ident of the cookies. You can use a spoon, but the bag method is easier and less messy.
Bake the cookies in the preheated oven for 9-11 minutes, or until the shortbread is light golden brown.
Remove raspberry cookies from the oven and allow them to cool on the pan for 2 minutes. Then, transfer to a wire rack to cool completely before serving.
Video
Notes
Chill the dough: To prevent the cookies from spreading too much, I recommend chilling the dough for at least 30 minutes before baking.
Make ahead of time: Prepare the cookie dough up to 24 hours ahead of time. Holiday baking is a lot easier when you can plan ahead.
Freezing the unbaked dough is not recommended. I froze a batch of dough and the cookies cracked when baked.
The red color of the raspberry jam makes these shortbread thumbprint cookies perfect for a Christmas or Valentine's Day treat. Feel free to use any flavor of jam you like. Other favorite flavors of mine are lemon (using lemon curd) and chocolate caramel thumbprints.