Preheat the oven to 350°F (175°C). Grease a muffin tin or line it with paper liners.
In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.
In a large bowl, cream together the softened butter, brown sugar, and granulated sugar until light and fluffy.
Beat in the egg and vanilla extract until well combined.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Stir in the rolled oats, ground cinnamon, diced apples, and nuts (if using) until evenly distributed.
Scoop about 2 tablespoons of the cookie dough into each muffin cup, pressing it down slightly to form a cup shape.
In a small bowl, mix together the granulated sugar and ground cinnamon for the topping. Sprinkle this mixture evenly over the cookie cups.
Bake for 15-18 minutes, or until the edges are golden brown and the tops are set.
Allow the cookie cups to cool in the pan for 5 minutes before transferring them to a wire rack to cool completely.