1-Preheat the oven to 350℉ and line: Preheat the oven to 350℉ and line a baking sheet with parchment paper. Set aside.
2-In the bowl of a stand mixer: 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.
3-Turn off the mixer and scrape down: Turn off the mixer and scrape down the insides of the bowl. With the mixer on low speed gradually incorporate the flour.
4-Optional, but recommended: Optional, but recommended: Chill the dough for an hour to prevent the cookies from spreading.
5-Using a small cookie scoop or spoon,: Using a small cookie scoop or spoon, portion and arrange ½ tablespoon-sized dough balls about 2 inches apart on the prepared baking sheet.
6-Make an indent in the center of: 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.
7-Fill each indent with jam: 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 indent of the cookies. You can use a spoon, but the bag method is easier and less messy.
8-Bake the cookies in the preheated oven: Bake the cookies in the preheated oven for 9-11 minutes, or until the shortbread is light golden brown.
9-Remove raspberry cookies from the oven and: 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.