Preheat your oven to 350°F (180°C) and line two baking sheets with parchment paper.

Separate the egg whites from the yolks. In a mixing bowl, beat the egg whites until stiff peaks form, gradually adding the sugar, one tablespoon at a time.

Add the egg yolks one at a time, beating well after each addition. Mix in the vanilla extract.

Sift the gluten-free flour directly into the egg mixture and gently fold it in with a spatula, being careful not to overmix. Transfer the batter into a piping bag fitted with a round tip, and pipe small circles onto the prepared baking sheets, leaving space between each.

Sprinkle the tops with poppy seeds before baking. Bake for about 10 minutes, or until the cookies are lightly golden around the edges. Once the cookies have cooled, spread berry jam on one cookie and sandwich it with another. This recipe yields about 25 sandwich cookies.