Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper.
In a large bowl, cream together the softened unsalted butter, granulated sugar, and light brown sugar until light and fluffy, about 3 minutes.
Add the egg and vanilla extract to the butter and sugar mixture and beat until fully combined.
In a separate bowl, whisk together the all-purpose flour, baking soda, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing just until combined.
Fold in the chopped pecans and crushed cornflakes cereal until evenly distributed throughout the dough.
Using a tablespoon, scoop the dough and place it onto the prepared baking sheets, spacing each cookie about 2 inches apart.
Gently flatten each cookie dough ball with the palm of your hand to about 1/4 inch thickness.
Bake in the preheated oven for 10 to 12 minutes, or until the edges are golden brown and the cookies are crisp.
Remove the baking sheets from the oven and allow the cookies to cool on the sheets for 5 minutes before transferring them to a wire rack to cool completely.