In a large bowl, cream the butter and sugar together using a hand mixer or stand mixer until soft and fluffy, about 2 minutes on medium speed.
Add vanilla and salt and continue to beat until fully incorporated. Gradually add flour, and mix until your mixture forms into a dough mass.
Use your hands to form the dough into a ball, and then shape it into a flat disc. Cover tightly in plastic cling wrap, and chill in the refrigerator for 30 minutes.
Preheat oven to 350 F, and line a half-sheet baking pan with parchment paper or a silicone mat.
Once the dough is chilled, lightly dust a clean surface with flour, and place the dough on top. Use a rolling pin to roll out the dough until it is ¼-inch thick. Use Linzer cookie cutters to cut out the cookies. On half of the cookies, use a smaller cookie cutter to cut shapes in the centre of the cookie. Remove the smaller cookie shapes and bake them as separate little shortbread cookies, or roll them together with the scraps to cut out more cookies.
Place the cookies half-inch apart on the lined baking sheet, and chill in the refrigerator for another 10 minutes, while the oven is preheating.
Bake the cookies for 8-10 minutes, or until the edges just start to brown. Allow the cookies to cool on the baking sheet for 5 minutes, then transfer them onto a wire cooling rack to cool completely to room temperature.
Then, dust confectioners' sugar on the cookies that have the centre cut out. Then, spread some raspberry jam on the bottom halves of each cookie (the cookies that do not have the centre cut out), leaving a small ⅛-inch border around each of the cookies. You can use a knife or use a piping bag with a round piping tip to pipe the jam onto the cookies.
Carefully place the tops of the cookies on the jammy bottoms. Be careful not to smudge the sugar on top. You can fill the open centres with more jam if desired.