In a large bowl, beat sugar and 1/2 cup butter with mixer at high speed until light and fluffy. Add flour, vanilla, salt, and egg; beat on low speed until blended, occasionally scraping bowl.
Shape dough into ball. Wrap and refrigerate 1 hour, until firm enough to handle.
To Make Filling: Stir remaining ingredients and 4 Tablespoons butter untl mixed. Set aside.
On waxed paper, roll half of dough into 10 by 8 inch rectangle. Spread dough with half of the poppy seed mixture. Starting at narrow side, roll jelly-roll fashion. Wrap and refrigerate 1 hour, until firm. Repeat with remaining dough and filling.
Preheat oven to 375 degrees F (190 degrees C).
Slice one roll cross-wise into 1/4 inch thick slices. Place 1/2 inch apart on ungreased cookie sheet. Bake 10 to 12 minutes, until lightly browned. Cool on wire rack and repeat with remaining dough.