Preheat the oven to 350 degrees F.
In a small mixing bowl, combine the flour, oats, baking powder, cinnamon and kosher salt.
In large mixing bowl or the bowl of a stand mixer, combine the brown sugar, granulated sugar and butter. Using an electric beater or the paddle attachment of the stand mixer, beat together the butter and sugar until it they are light and fluffy. Add the eggs one at a time and beat them into the butter and sugar mixture. Add in the vanilla as well.
With a rubber spatula or with the stand mixer on low, gradually add the flour mixture into the butter/sugar mixture. Mix until just combined. Fold in the chocolate chunks and walnuts.
Spoon the cookie dough by 2 tablespoon-size balls onto an ungreased cookie sheet. Place the cookie dough balls 2 inches apart on the cookie sheet. Place the cookie sheet in the oven and bake for 12 to 13 minutes.
When the cookies come out of the oven IMMEDIATELY sprinkle each cookie with a few grains of sea salt. This is really important to do while the cookies are hot.
Let the cookies cool for 2 to 3 minutes and then transfer them to a cooling rack.