01 - Preheat the oven to 350°F. Line two baking sheets with parchment paper.
02 - Whisk together the flour, baking powder, baking soda, salt, cinnamon, and nutmeg in a medium bowl.
03 - Beat the unsalted butter and granulated sugar until light and fluffy in a large bowl.
04 - Add the egg and vanilla extract to the butter mixture and beat until fully combined.
05 - Gradually add the dry ingredients to the wet mixture, stirring just until incorporated.
06 - Gently fold the diced apples and chopped nuts, if using, into the dough.
07 - Drop rounded tablespoons of dough onto the prepared baking sheets, spacing each about 2 inches apart.
08 - Bake for 11 to 13 minutes until edges are lightly golden and centers are set.
09 - Allow cookies to cool on the sheet for 5 minutes before transferring to a wire rack to cool completely.
10 - Whisk powdered sugar, milk, and vanilla extract until smooth and thick but pourable.
11 - Drizzle or spread frosting over the cooled cookies. Let the frosting set before serving.