PREHEAT oven to 375º F. Paper-line or grease 18, 2 1/2 –inch muffin cups.
COMBINE all-purpose flour, whole-wheat flour, baking powder and salt in small bowl. Beat butter, peanut butter and brown sugar in large mixer bowl until creamy. Add eggs and milk; beat until smooth. Add flour mixture to peanut butter mixture; beat until just blended. Stir in morsels. Spoon batter into prepared cups, filling 3/4 full.
BAKE for 18 to 20 minutes or until wooden pick inserted in centers comes out slightly sticky. Cool in pans on wire racks for 5 minutes; remove to wire racks to cool completely. Store in airtight containers.