Preheat oven to 350°F. Grease or spray with nonstick cooking spray a 13 x 9 x 2-inch pan or a 15 x 10 x 1-inch jelly-roll pan.

For cookie dough: In stand mixer bowl fitted with the paddle attachment, or a handheld electric mixer, beat butter and brown sugar until light and fluffy. Mix in eggs and vanilla.

In a medium bowl, whisk flour, salt and baking soda. Stir in oats. Gradually add to bowl mixture and mix until well combined. Set aside.

For filling: In saucepan, melt chocolate chips, condensed milk and butter; gently stir until chocolate chips have melted. (This step can also be done in the microwave stirring every 20 seconds.) Stir in pecans and vanilla.

Press two-thirds of dough evenly in the prepared pan. Cover with fudge filling. Crumble the remaining dough over the top.

Bake 25 - 30 minutes, or until golden. Transfer the pan to a wire rack to cool. When cool, cut into bars.