These take a bit of time, but are well worth it!
1. Cream butter and sugar. Beat in peanut butter, vanilla and egg. Add baking soda and salt, stir until well distributed, then add flour and stir until it is absorbed. Chill dough for 45 minutes (no longer).
2. Meanwhile prepare the filling. Melt chocolate with butter in a bowl over hot (not boiling) water. Do not cook any longer than to melt it, as it will dry out the chocolate. Let cool slightly.
3. Divide chilled dough in half. On a lightly floured surface roll each half into 10 X 8 inch rectangle and spread each rectangle with half the chocolate mixture, stopping about ½ inch from sides.
4. Starting from long side, roll up each rectangle jelly roll style to form 2 long rolls. Wrap separately seam side down in plastic wrap. Chill until firm (at least 3 hours).
5. Using a sharp knife, cut into 1/4 inch slices. Place 1 1/2 inches apart on a lightly greased baking sheet. Bake at 375 degrees for 10-12 minutes or until lightly browned. Transfer to wire rack to cool.