the goods:
1/2 teaspoon baking soda
1/4 teaspoon salt
2 cups plus 2 tablespoons all-purpose flour
1/2 cup (1 stick) unsalted butter, room temperature
1 cup packed light-brown sugar
1 and 1/2 cups smooth peanut butter
1 large egg
1/2 cup whole milk
1/2 cup granulated sugar
In a bowl, whisk together baking soda, salt, and 2 cups flour; set aside. In a large bowl, using an electric mixer, beat butter, brown sugar, and 1 cup peanut butter until light and fluffy; beat in egg. With mixer on low, gradually add flour mixture, beating just until combined (do not overmix).
Form dough into two 8-inch-long rectangular logs. Wrap each log in wax or parchment paper; freeze until firm, about 1 hour.
Preheat oven to 375 degrees. With a sharp knife, slice dough 1/4 inch thick; place on a baking sheet, 1 inch apart. Bake until cookies are puffed, 12 to 14 minutes. Transfer to wire racks, let cool. Repeat with second log.
In a small saucepan, whisk milk and 2 tablespoons flour over medium until thickened, 2 minutes (it will continue to thicken after removed from heat); let cool. In a bowl, using mixer, beat together 1/2 cup peanut butter and granulated sugar; beat in cooled milk mixture. Spread smooth sides of half the cookies with peanut butter filling; sandwich remaining cookies.
