Beat together butter and peanut butter until light. Gradually beat in sugar, beating until light and fluffy. Beat in egg yolk and vanilla to blend thoroughly.
Sift together flour and salt. Add to creamed mixture; mix to a smooth dough.
Fit desired plate into cookie press. Put one fourth of the dough in cookie press at a time. Force cookies 1" apart onto ungreased baking sheet Bake in moderate oven (375°) 8 to 10 minutes, or until delicately browned. Remove cookies and cool on racks.