Beat butter until fluffy, beat in sugar. Beat in eggs and vanilla. Slowly stir in flour, baking powder and salt until blended. Divide dough into 4 equal balls, flatten into disks. Wrap and refrigerate overnight.
Roll out each disk into a 14x5 1/2 inch rectangle, 1/8 inch thick. Spread 3 T jam over each rectangle, leaving a 1/4 in border all around. Starting with long side, roll up.
Place 2 rolls on greased large cookie sheet, leaving room between them, as they will spread and flatten out.
Bake in preheated 350 oven for 20 minutes or until golden brown. Repeat with remaing 2 rolls. When cool, slice each roll diagonally into 12 pieces.
Makes 4 dozen