Toast the pecans: Preheat the oven to 350°F. Spread the pecans on a baking sheet and toast for 5–7 minutes until fragrant. Let cool, then finely chop.
Mix the wet ingredients: In a large bowl, whisk together the melted butter, brown sugar, and granulated sugar. Add the egg, maple syrup, and vanilla, mixing until smooth.
Add the dry ingredients: Stir in the flour, cornstarch, baking soda, baking powder, salt, and cinnamon until just combined.
Fold in the pecans: Gently fold in ½ cup of the finely chopped pecans, reserving the remaining ¼ cup for topping.
Scoop and bake: Scoop 2-tablespoon portions of dough onto a parchment-lined baking sheet, spacing them 2 inches apart. Bake at 350°F for 10–12 minutes, until the edges are set and lightly golden.
Cool: Let the cookies rest on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
Make the glaze: Whisk together the powdered sugar, maple syrup, melted butter, vanilla, and salt. Add milk, 1 teaspoon at a time, until thick but spreadable.
Decorate: Spread or drizzle the maple glaze over the cooled cookies. Sprinkle with the reserved toasted pecans and flaky sea salt, if desired. Let the glaze set before serving.