Into a food processor add ⅓ cup brown sugar and hazelnuts. Pulse several times until you have a fine crumb. Set aside.
In a medium bowl, whisk together the flour, baking powder, cardamom, cinnamon and salt. Set aside.
In a large bowl, cream together the butter and remaining ⅓ cup brown sugar until light and creamy, about 2 minutes. Add eggs and vanilla and beat to combine. Add half of the hazelnut and flour mixtures into the butter. Beat on low speed until just combined and then add the other half of the nuts and flour. Blend until dough just comes together.
Divide dough into two equal pieces. Roll into a ball and flatten each ball into a disc about 1/2" thick. Wrap with plastic wrap and refrigerate 3 hours or overnight.
ROLL AND CUT THE CUT THE COOKIES
Preheat the oven to 350°. Line two baking sheets with parchment paper or silpats. Set aside.
Generously flour a work surface and place one disc of dough in the center. Lightly flour the top of the dough and your rolling pin. (Keep the other dough in the refrigerator until you're ready to use it.)
Roll the dough out to about ⅛"-¼" thick with the rolling pin. Turn the dough a quarter of a turn after each roll to prevent sticking. (note: you may need to add a little more flour to the underside of the dough.)
Use a 2" cookie cutter to cut out 20 cookies (you may need to collect scraps and roll out the dough again to achieve 20.) Transfer the cookies to a prepared sheet pan and refrigerate for 10 minutes before baking to prevent spreading. These are the bottoms of the cookies. Bake the cookies for 11-14 minutes or until lightly browned on the edges. Remove from the oven and cool to room temperature.
Remove the second disc of dough from the refrigerator and roll it out in the same manner as the first. Cut out 20 cookies from the dough and place them on a prepared baking sheet. Use a 1" heart-shaped cookie cutter and press it into the center of each cookie. These are the "tops" of the spice Linzer cookies. Refrigerate 10 minutes before baking to prevent spreading. Bake the cookies for 11-14 minutes or until set and lightly browned on the edges. Remove from the oven and cool to room temperature.
Transfer the baby hearts to another prepared baking pan (or just fit them around the tops) to bake (those are for you -- because you need a taste tester or 20). Refrigerate 10 minutes before baking to prevent spreading.
Place the powdered sugar in a fine mesh sieve. Hold the sieve over the "top" cookies with the heart center. Gently tap the side of the sieve to coat the cookies with sugar.
To make the filled cookies, hold a "bottom cookie" and turn it over to the side that had contact with the pan. Spread a thin layer of jam over the cookie and spread it out slightly. Top with a powdered sugar coated "top" cookie.
Continue in this manner until all the cookies are filled.