In the bowl of a heavy-duty mixer fitted with the paddle attachment, cream together butter and sugar on medium speed until light in color and fluffy, 3 to 4 minutes. Add orange zest, orange juice, vanilla extract, and 2 eggs, one at a time, scraping down the sides of the bowl when necessary
Sift together flour, baking powder, and salt. With the mixer on low speed, slowly add to butter mixture until just combined. Divide dough into thirds. Wrap tightly in plastic, and refrigerate for at least 1 hour.
Heat oven to 350 degrees. Remove one-third of the dough from the refrigerator at a time. On a liberally floured surface, roll dough to a 1/8-inch thickness. With a 3-inch fluted round cutter, cut out as many circles as possible, and place on a prepared baking pan. Chill until firm, about 30 minutes.
Meanwhile make the poppy-seed filling: Grind poppy seeds in a food processor. Place seeds in a medium saucepan. Add milk and honey. Cook over a medium-low heat until thickened, stirring occasionally, about 20 minutes. Add raisins and lemon zest. Remove from heat. Cool completely before using.
Remove chilled circles from refrigerator. Place each filling into a medium pastry bag fitted with a coupler. Pipe 2 teaspoons of filling into the center of each circle. Whisk together remaining egg with 1 teaspoon of water. Brush edges with egg wash. Fold in sides to form a triangle. Pinch dough to enclose the filling.
Bake the cookies in the center rack of the oven until golden, 12 to 15 minutes. Cool cookies on baking sheets for 5 minutes; then transfer to wire racks to cool completely.
You can substitute apricot preserves for the poppy-seed filling.