Cream together butter and sugar in a large bowl with an electric mixer until smooth. Beat in eggs one at a time, then stir in orange juice and vanilla until well combined. Mix in baking powder, then gradually stir in flour until dough forms a ball. Cover and refrigerate dough for at least 2 hours.

Preheat the oven to 375 degrees F (190 degrees C). Grease baking sheets.

Roll dough out on a lightly floured surface to 1/4-inch thickness. Use a cookie cutter or drinking glass to cut dough into 3-inch circles, then place on the prepared baking sheets. Spoon 1 teaspoon poppyseed filling onto the center of each circle. Pinch the sides of each circle to form a triangle, covering as much of the filling as possible. The cookies may be chilled on the baking sheets before baking if desired to help retain their shape.

Bake in the preheated oven until light golden brown, 8 to 10 minutes. Cool on the baking sheets for a few minutes before removing to wire racks to cool completely.