In a large bowl, cream together your butter and sugar together until smooth and the sugar is fully dissolved. Add in your egg yolks and vanilla extract. Whisk to combine everything together.
Add your flour, cornstarch, and salt to the butter / egg mixture. Mix until you get a crumbly dough that holds its shape when squeezed together. Transfer the cookie dough to a large piece of plastic wrap. Press the dough down and shape into a disk. Wrap the dough and refrigerate for about 1 hour, or until it firms up. You can also place the dough in the freezer for 30 minutes to make it quicker. Preheat your oven to 350°F. Line 2 large baking sheets with parchment paper, and set aside. Take the dough out of the fridge, unwrap, and place on a lightly floured work surface. Roll the dough out to a 1/4 inch thick. Use a lightly floured, 2 inch round cookie cutter to cut out your cookie shapes. Use a smaller cookie cutter (about 3/4 inch) to cut holes in the center of half of your cookies for the sandwich tops.
Arrange the cut cookies on your baking sheets, leaving about 1 inch of distance between each cookie. ** (see notes) Repeat the rolling and cutting process with the remaining dough. Bake your cookies in the center of your preheated 350°F oven for 10 to 12 minutes, or until just slightly golden around the edges.
Let the cookies cool for about 5 minutes on your baking sheet. Transfer the cookies to a wire rack to cool completely to room temperature before sandwiching. Spread around 1 teaspoon of your jam on one of your cookies that don't have a hole in the center. Top your jam with one of the cookies with a center cutout. Gently press to push the jam to the edges. Repeat with the remaining cookies until they're all sandwiched.
(Optional) Top your cookies with a dusting of powdered sugar before serving.