Grease a 9-inch springform pan and line the bottom with parchment paper.

Preheat oven to 300°F (150°C).

Grind poppy seeds in a coffee grinder in batches for about 20 seconds until sticky.

Combine ground poppy seeds with potato starch in a large bowl.

In a separate bowl, beat egg yolks until slightly thickened.

Slowly add sugar and vanilla to the egg yolks, mixing until combined.

Slowly pour in melted and cooled butter, then add the poppy seed mixture. Beat until combined.

In a clean bowl, beat egg whites until soft peaks form.

Gently fold the egg whites into the batter.

Pour the batter into the prepared springform pan.

Bake for 50-60 minutes, or until a toothpick inserted into the center comes out fairly dry.

Cool on a wire rack for one hour before unmolding.

Dust with powdered sugar before serving.