Chocolate–Peanut Butter Thumbprints

In medium bowl, whisk flour, baking soda, and salt. With mixer on medium speed, beat butter and sugar until creamy. Add egg, vanilla, and peanut butter; beat until well mixed. Add flour mixture; beat on medium-low speed just until well combined. Arrange oven racks in top and bottom thirds of oven. Preheat oven to 350 degrees F. Form dough into 1-inch balls; roll in chopped unsalted roasted peanuts. Place balls on 2 large cookie sheets, spacing 2-inchs apart. With floured handle end of wooden spoon, make indentation in center of each ball. Bake 15 minutes or until golden around edges, switching racks halfway through. Cool on sheets on wire racks. If indentations have puffed, press down while cookies are warm. When cookies have cooled, fill indentations with a teaspoon of melted milk chocolate. Refrigerate until chocolate has set. Once chocolate has set, store cookies in airtight container in refrigerator up to 1 week or in freezer up to 2 weeks.