Ground desiccated coconut: Place the desiccated coconut into a small food processor. Pulse it in short bursts until it reaches a finer texture. Be careful not to over-process, as it could turn into coconut butter.
Whisk together the flour, salt, desiccated coconut, and baking powder in a medium-sized bowl. Set aside.
Add the softened butter and sugar to a large mixing bowl. Use a hand mixer or a stand mixer fitted with the paddle attachment to whip on medium speed until light and fluffy, about 3-4 minutes. Add the egg and vanilla extract, and beat on high speed until well combined. Scrape down the sides and bottom of the bowl, then beat again as needed to fully incorporate.
With the mixer on low, gradually add the flour/coconut mixture into the butter mixture and mix until just combined, forming a dough mass. If the dough looks too dry or stiff for your mixer, turn it out onto a countertop and finish kneading by hand.
Press the dough together until you have a smooth ball. Divide the dough in half, shape each half into a 1-inch disc, cover with plastic wrap, and chill until firm, for at least 1 hour.
Once the dough is chilled, preheat the oven to 320°F (160°C) and line two baking sheets with parchment paper.
Remove one piece of dough from the refrigerator and let it sit for 5-10 minutes until it’s soft enough to roll but still cold.Lightly dust a clean surface with flour, then place the dough on top. Use a rolling pin to roll out the dough to a 1/4-inch thickness, ensuring it’s evenly thin throughout. Move the dough around and check underneath frequently to make sure it’s not sticking. Add more flour if the dough becomes too sticky. Use a 2½-inch cookie cutter (star, round, or heart-shaped) to cut out the cookies. You should end up with about 20 cookies. Keep in mind that you'll need two pieces for each Linzer cookie — one for the bottom and one for the top.
Arrange cookies on prepared baking sheets about 1-inch apart.
Bake cookies on the middle rack of your oven, one tray at a time for 10-12 minutes or until lightly golden around the edges.
Let the cookies cool on the baking sheet for 5-10 minutes before transferring them to a wire rack to cool completely before filling. (You may need to bake the cookies in batches, so make sure to let the pans cool completely between batches.) Re-roll the off-cuts and cut out as many more cookies as you can (you’ll need an even number). Repeat with the remaining dough.
Place the cooled cookies with the center cutouts on a cooling rack with a baking tray underneath. Dust them with powdered sugar. These will be the tops of your cookies.
Then, spread a thin layer of jam into the center of the bottom halves of each cookie (the cookies that do not have the center cut out). You can use a knife, the back of a spoon, or a piping bag with a round piping tip to pipe the jam onto the cookies. Place the powdered sugar-dusted cookies on top, pressing gently to form a sandwich. Be careful to only touch the sides of the top cookie, not the part covered in powdered sugar. Enjoy!