Mini Peanut Butter Chocolate Chip Cookie Cups

unsalted butter, softened 1 large egg 3/4 cup brown sugar, packed 1/2 cup creamy peanut butter 1/4 cup white sugar 1 tbsp vanilla extract 1 1/2 cups flour 1/2 tsp baking soda 1/4 tsp teaspoon salt 1 1/2 cups semi-sweet chocolate chips