In a large bowl whisk flour with salt, 1 1/2 tablespoons sugar, and lemon zest.
Dissolve the yeast with remaining sugar in lukewarm with milk, let it stand for few minutes, and pour it over flour mixture. Add beaten egg, egg yolk, melted butter, and vanilla extract and knead the dough until smooth, for about 5 minutes. Cover and let it rise at room temperature for about 1 hour or until doubled in size.
On a floured surface divide the dough into 4 pieces. Flatten each piece of dough using your hands and fill each with about 1 tbsp jam. Seal well and form each piece into a ball. Place the dumplings into a floured surface, cover with a kitchen towel, and let them rest for 30 minutes.
In a small bowl combine the ground poppy seeds with powdered sugar to have them ready for serving.
Bring to a boil approximately 3/4 cup water. Use a greased steamer basket and place it in the pot. Add the dumplings, cover the pot with a lid, and let them steam for about 15-17 minutes.
Serve the dumplings immediately. Pour vanilla sauce (or melted butter) and poppy seed mixture on top.