For the citrus curd:

1. Bring a small pot, with a few inches of water, to a boil. In a medium sized heat-proof glass or stainless steel bowl (one that can sit atop the pot of water) whisk together the citrus juice, whole egg, egg yolks, sugar, and salt. 
2. Keeping the pot of water at a simmer, on medium heat, place the bowl on top of the pot– like a double broiler. Using a rubber spatula, cook the mixture, stirring constantly, until it is very thick– thick enough that your spatula leaves a trail through the curd. Be patient, this might take up to 25 minutes.
3. Once thickened, remove from the bowl from the heat, and stir in the butter. Gently press a piece of plastic wrap over the top of the curd (this will prevent a layer of film from forming), and refrigerate overnight, or until fully cooled.

For the cookies:

1. In a large bowl, beat together the butter, sugars, orange zest and vanilla (with the paddle attachment, if using a stand mixer), until light and creamy—about 3 minutes. 
2. Add the flour, cardamom (if using), and salt. Mix on low speed, until the dough starts coming together into moist clumps.
3. At this point, you could press the dough into a disc, wrap in plastic wrap and refrigerate (or freeze) for a later day. Let it soften slightly before rolling out.
4. I like to roll out the dough immediately after making, between two sheets of parchment paper– roll it out to ¼ inch thickness. Place the rolled out dough, on the parchment, onto a large surface (a hard placemat, cutting board, cookie sheet), and freeze for at least 20 minutes.
5. Preheat the oven to 350 F, and line two cookie sheets with parchment. Using your cookie cutter (or the rim of a cup), cut out an even number of cookies (you should get 28-34, with a cutter that is 1.5-2 inches in diameter). Using a smaller cookie cutter, or back of a piping tip, cut a small hole from the center of half the cookie cut-outs. 
6. Place the cut-outs on your cookie sheets, about 1-2 inches apart; they won’t spread too much. Bake for 8-11 minutes, or until golden on the bottom. Allow them to cool completely before filling. 

Assembly

1. Dust the tops of the cooled cookies with powdered sugar. Flip over the bottom halves, add a spoonful of curd on top of each (about 2 teaspoons, or however much you desire). Place the cut-out cookies on top. Enjoy!

The filled cookies will stay fresh for a few days, in an airtight container, in the fridge. Any longer and they may start to soften.