Make the cookies: Process flour, sugar, and salt in a food processor for a few seconds until combined. Add butter and pulse several times until mixture becomes crumbly and resembles coarse meal. Add egg yolk, lemon zest, and vanilla, and keep pulsing until dough starts to clump together. Do not process to the point that a large ball of dough is formed; rather, the dough should be quite crumbly with large clumps. If dough is too dry, add water, 1-2 teaspoons at a time. Another way to check if the dough is done is to take a piece of dough and press it between your thumbs – the dough should stick well together without feeling dry. Avoid overworking the dough throughout the process.
Turn the dough to a floured surface and form into a ball. It should come together easily without being sticky. Flatten the ball slightly with your hands (for easy rolling later on) and wrap with plastic wrap. Refrigerate for at least 1 hour, or up to 3 days.
Make the filling: In a medium saucepan, place poppy seeds, milk, butter, sugar, honey, and salt and cook over medium-low heat, stirring occasionally, for about 10 minutes or until the seeds absorb the milk and the mixture has thickened. Remove from heat. Add lemon zest, nuts, and raisins, and mix to combine. Let cool to room temperature before using.
Preheat oven to 350F/180C. Line 2 pans with parchment paper and set aside.
Roll out the dough on a floured surface until it’s about 1/8-inch (4mm) thick. If the dough is too crumbly and breaks, leave it for a few minutes to soften. Cut rounds of dough using a 3-inch (8 cm) cutter or a glass.
Transfer rounds gently using a thin spatula to the prepared baking sheets, placing them 1-inch (3cm) apart. Put 1 teaspoon of filling in the center of each round, then fold up the edges to form a triangle, pinching the corners together tightly to prevent filling from leaking while baking.
Bake for about 15 minutes or until the cookies are golden just at the edges. Let cool in the pan for 5 minutes, then transfer to a cooling rack to cool completely. Sift powdered sugar on top.
Store cookies at room temperature in an airtight container for up to 5 days.