Preheat the oven to 350℉ and prepare a parchment lined baking sheet.
Using a stand mixer fitted with the paddle attachment or an electric hand mixer and large bowl, cream the butter on High speed for about 1 minute. Add the sugar and egg yolk, then mix again to combine for 1 minute.
Scrape down the insides of the bowl. Add the flour and mix on low speed to incorporate.
Optional, but recommended: Chill the dough for an hour to prevent the cookies from spreading.
Portion dough into ½ tablespoon balls.
Place on a parchment lined baking sheet. The cookies do spread a bit, so leave room.
Make an indent in the center of the dough with your thumb OR grease the back of a 1/2 teaspoon measuring spoon with cooking spray and use that to make your idents.
Fill the cookies with jam. I recommend adding jam to a ziploc 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.
Bake the cookies at 350 for 9-11 minutes, or until the shortbread is light golden brown.
Remove raspberry thumbprints from the oven and allow the cookies to cool on the pan for 2 minutes. Transfer them to a wire rack to cool completely before serving.
Video
Notes
HELPFUL TIPS FOR THUMBPRINT COOKIES:Indents: Be GENTLE when pressing into the cookies to prevent them from cracking. Use your thumb, or if you want more uniform indents use the back of a teaspoon or the bottom handle of a wooden spoon to get perfect indentsPrevent Cookies from Spreading/Cracking: We recommend chilling your dough for one hour to prevent the cookies from spreading and cracking. If the cookies crack when you make your indents, just gently fix them.Fixing your cookies: This dough is pretty pliable. If your cookies start to crack, just fix them gently before baking. If you spill jam on the side, just wipe away with a napkin before baking!Measure your ingredients correctly - It is super important to measure your ingredients correctly, specifically the flour! Check out this tutorial on how to measure flour correctly.Watch when baking! These go from perfectly golden brown to burnt quickly!Heat the jam: Heating the jam for 10 seconds in the microwave makes it easier to handle