1. Preheat oven to 375°F. Whisk flour, baking powder, salt and spices together. In a bowl, beat butter with brown sugar until creamy, about 3 minutes. Beat in molasses and egg. Stir in flour mixture.
2. Form dough into 1-inch balls. Place 2 inches apart on ungreased baking sheets. Flatten slightly. Bake until centers are still slightly soft, about 12 minutes. Transfer to wire racks to cool completely.
3. Beat confectioners' sugar, milk, peanut butter and vanilla until creamy. Spread cookies with glaze; let stand until set.