Add hazelnuts to a medium skillet over medium heat. Toast for 1-2 minutes until fragrant and warm to the touch. Pour in maple syrup and vanilla extract and immediately stir to combine. Toast for 30 seconds more then transfer to a plate. Set aside.

In a large mixing bowl, whisk together butter, sugars, milk, almond butter, and vanilla until creamy. Add flour, oats, corn starch, baking soda, and salt. Use a wooden spoon or rubber spatula to fold dry ingredients into wet until a soft dough forms.

Roughly chop cooled hazelnuts and add to dough along with chopped chocolate. Fold into dough to combine. Cover and refrigerate for at least 30 minutes.

Preheat oven to 350°F. Use a 1.5 tbsp cookie scoop to distribute dough on to a lined baking sheet. Top with additional chopped chocolate. Bake for 12 minutes until tops are light golden brown. The centers may feel a bit underdone, but they will continue to bake out of the oven. Allow cooling on the baking sheet for 10 minutes before transferring to a wire cooling rack. Finish with flaky sea salt and enjoy!