Preheat oven to 350º F. Line a 9×13 baking pan with parchment paper leaving overhang (parchment paper sling). Spray exposed areas of the pan with baking spray that contains flour.
In a medium-sized bowl whisk together the all-purpose flour, baking soda, and kosher salt. In a large bowl beat the butter and brown sugar together, until light and fluffy. Add the eggs and vanilla extract, mixing until well combined.
2 1/2 cups All-purpose flour, 1 tsp Baking soda, 1/2 tsp Kosher salt, 1 cup Unsalted butter, 2 cups Dark brown sugar, 2 large Eggs, 2 tsp Vanilla extract
Stir in the dry ingredients and oats until just combined.
3 cups Old-fashioned oats
Press 2/3 of the cookie dough (about 3 1/2 cups) into the bottom of the pan. Reserve the remaining oat mixture for the top.
In a microwave-safe bowl combine the semi-sweet chocolate chips and condensed milk, and microwave in 30-second intervals, stirring the chocolate mixture between intervals, until almost melted. Stir in the vanilla until it’s smooth and creamy.
2 cups Semi-sweet chocolate chips, 1 14oz Can of sweetened condensed milk, 2 tbsp Unsalted butter, 1/2 tsp Vanilla extract
Pour the fudge mixture over the cookie dough and spread evenly with an offset spatula. Drop small pieces of the remaining cookie dough over the top of the chocolate fudge layer.
Bake for 20-23 minutes, until the top cookies are golden brown. DO NOT OVERBAKE. Cool completely before slicing, for 3-4 hours.