Make the pectin mixture: Combine the sugar and pectin in a medium bowl and stir until well mixed.
Make the berry mixture: Sprinkle the pectin mixture evenly over the berries and stir until incorporated.
Make sure to sprinkle the pectin mixture evenly, or it will clump.
Cook the jam: Transfer the berry mixture to a medium saucepan, cover, and bring to a simmer over medium heat. Cook, stirring occasionally and adjusting the heat as needed if the mixture threatens to boil over, until the berries have broken down and the mixture is thick and jammy, 30 to 45 minutes. Remove from the heat. If you prefer your jam to be seedless, strain it through a fine-mesh sieve before letting it cool.
Use or store the jam: If you’ll be using the jam immediately or within a few days, let it cool completely;
otherwise, process the jam for long-term storage while it’s still hot.
Roll the dough slightly thicker than a tart shell, 1/4 inch (6 mm) thick. Use your favorite cookie cutter to cut shapes from the dough. With a smaller cookie cutter, cut a smaller shape from the center of half the cookies. Arrange all the cookies on a parchment paper–lined sheet pan and bake in a preheated 350°F (175°C) oven for 10 minutes. Let cool completely in the pan. Turn the cookies without the center hole bottom-side up and dollop a teaspoon of jam onto each. Dust the cookies with the center hole with confectioners’ sugar and place on top of the jam. Push down slightly to make a sandwich. Once assembled, the cookies can be stored in an airtight container in a cool, dry place for up to 2 days.