In a large bowl, whip egg whites until stiff peaks form.

Add 6 tbsp sugar, one by one. Beat until smooth. Add vanilla extract and lemon zest.

Add 6 egg yolks, one by one. In the end, carefully fold in ground pumpkin seeds, stirring with a spoon (do not use mixer for this step).

Pour batter into a 12x12 inch (33x33 cm) sheet/jelly-roll pan covered with baking paper. Bake at 350 F (175 C) for about 10 minutes.

Take out of the oven and immediately roll cake (together with baking paper), beginning at the narrow end. Place the cake on a wire rack, seam-side down. Let cool.

Unroll cold sponge cake, peel away the backing paper from the sponge and spread the cake with sour cream. Roll up again, seam-side down, and put in a refrigerator for at least 2 hours. Serve lightly dusted with powdered sugar.