Prepare for baking. Preheat your oven to 350°F (175°C). Then, line a baking sheet with parchment paper or a silicone baking mat to prevent sticking and make sure even baking.
Cream the wet ingredients. In the bowl of a stand mixer, cream together the butter and brown sugar on medium speed until smooth and fluffy, about 2-3 minutes. Then, add the eggs one at a time, mixing well after each addition to make sure a smooth consistency. Next, mix in the vanilla extract until just combined.
Mix the dry ingredients. In a separate bowl, whisk together the flour, baking soda, cream of tartar, and salt until evenly combined. Then, slowly add the dry ingredients to the butter mixture, mixing at low speed and scraping down the sides until a soft dough forms.
Shape the cookies. Scoop out dough and roll it into walnut-sized balls. Then, place each dough ball on the prepared baking sheet, spacing them about 2 inches apart to allow for spreading. Use the bottom of a glass dipped in granulated sugar to gently flatten each dough ball to about ¼ inch thickness.
Bake. Bake in the preheated oven for 10-12 minutes or until the edges are lightly golden and the tops are set. For a slightly crispier cookie, bake an additional 1-2 minutes. Remove from the oven and let the cookies cool on the baking sheet for a few minutes before transferring them to a wire rack to cool completely.
Assemble the cookie sandwiches. Once the cookies are cooled but still slightly warm, spoon about 1 teaspoon of jam onto the flat side of half the cookies. Top with the remaining cookies, pressing gently to spread the jam and create a sandwich. If your kitchen is warm, place the assembled cookies in the refrigerator for 10-15 minutes to help the jam set.