Preheat the oven to 350F. Line two baking sheets with parchment paper.
Make the maple walnuts: Place a pan over medium heat. Add the walnuts, maple syrup, salt, and cinnamon. Stir until the walnuts are well coated. Continue to stir as the walnuts roast and the maple syrup has thickened and the nuts are fragrant. Transfer to a bowl to cool completely.
Make the chocolate chip cookies: Add the butter, brown sugar, and granulated sugar. Mix until well combined, about 2 minutes.
Add the egg, egg yolk, and vanilla extract. Mix until fully combined, about 1 minute.
Add the flour, cornstarch, salt, baking soda, and cinnamon. Mix on low until just combined.
Once the walnuts are completely cooled, chop them into smaller pieces. Add the chopped walnuts and chocolate chunks to the dough. Mix on low until just evenly distributed through the dough.
Use a large (1/4 cup) cookie scoop to scoop out the dough. Optionally, add an extra chunk of walnut and chocolate to the top of each dough ball, pressing gently to adhere.
Place up to 6 dough balls on each baking sheet. Bake for 10-14 minutes, until the edges are just set.
Top each cookie with a pinch of flaky salt. Let the cookies cool on the baking sheet for 10 minutes, then transfer to a cooling rack to cool completely. Enjoy!