• Sift the flour into a large mixing bowl. Add the unsalted butter, granulated sugar, egg yolk, sour cream, and salt.

  Using your hands or a pastry cutter, gently combine the ingredients, then knead the dough by working from the sides toward the center and from the bottom up until smooth dough forms, about 5 to 7 minutes.
• Divide the dough into two equal portions. Gently flatten each into a round disc and wrap tightly in plastic wrap. Chill in the refrigerator for at least 2 hours.
• When ready to cut out the cookies, remove the dough from the refrigerator and let it sit at room temperature for about 10 minutes before rolling.
• Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper.
• Roll out the dough to 1/4 inch thickness on a lightly floured surface using a floured rolling pin. Add more flour as needed if the dough becomes sticky.
• Cut the cookies using a 2-inch round Linzer cookie cutter. Then, use a smaller cutter to cut out the centers of half of the cookies (half of the cookies should have a shape cut out in the center, while the other half should remain plain).
• Use an offset spatula to transfer the cookies to the parchment-lined baking sheet, spacing them about 1 inch apart.
• Repeat with the second dough disc. Re-roll the dough scraps and continue cutting out more cookies until all the dough is used.
• Bake the cookies for 10 minutes, or until lightly golden and the edges are set.

  Remove the baking sheet from the oven and let the cookies rest for a few minutes. Then, transfer the cookies to a wire rack to cool completely to room temperature.
• Transfer the cut-out window cookies back to the baking sheet and use a fine mesh strainer to dust them with powdered sugar.
• Next, spread a dollop of cooled lemon curd on the base cookies and gently place a window cookie on top to form Linzer cookie sandwiches.

Make the Lemon Curd:

• Add enough water to a medium saucepan to come about 1 inch up the side.

  Bring to a simmer over medium-high heat. Once the water begins to boil, reduce the heat to low to maintain a simmer.
• Place the egg yolks, granulated sugar, lemon juice, lemon zest, and salt into a small heatproof glass bowl. Set the bowl on top of the saucepan (it should be large enough to fit on top without touching the water).
• Using a silicone whisk, whisk until the mixture is completely blended, then continue whisking as the curd cooks. Whisk and cook until the mixture thickens, about 10 minutes. If the curd isn't thickening, increase the heat and whisk constantly.

  Take the bowl off the heat and whisk the butter into the curd. The butter will melt from the heat of the curd as you whisk.
• Pour the curd into a clean jar and place a piece of plastic wrap directly on the surface, making sure it touches the top to prevent a skin from forming. Let it chill completely, then transfer to the fridge. The curd will continue to thicken as it cools. Refrigerate for up to 5 days. Enjoy!