Beat butter, sugar and peanut butter in large bowl until creamy. Add eggs and vanilla; beat well. Stir together flour and baking soda; add to butter mixture, blending well. Stir in peanut butter chips. Shape dough into 1-1/2 inch balls; roll in pretzel pieces, lightly pressing pretzels into dough. Place on ungreased cookie sheet. Lightly sprinkle unbaked cookies with salt, if desired.
Bake 12 to 13 minutes or until light brown around the edges. Cool 1 minute on cookie sheet. Remove to wire rack; cool completely. Makes about 48 cookies.