Line an 8" square baking pan with parchment paper, leaving overhang on the borders to lift it out. Preheat the oven to 350°F (180°C).

In a large bowl or stand mixer, combine the butter, brown sugar, granulated sugar, and salt. Beat together well with an electric mixer until creamy and smooth.

Add egg and vanilla extract and beat until creamy.

Add rolled oats and mix well. Sift flour, baking soda, and cornstarch into the cookie dough. Mix until combined. It will form a thick cookie dough.

Set aside about half a cup of dough. Press the rest of the dough into an even layer into the bottom of the lined baking pan. Set it aside while you work on the fudge layer.

Combine chocolate chips, sweetened condensed milk, cream, vanilla, and salt in a small saucepan.

Heat on low heat, stirring frequently to prevent the fudge from burning. Stir with a spatula until it becomes completely smooth.

Immediately remove the fudge from the heat and pour it into the prepared crust. Spread the fudge out into an even layer.

Top the fudge with flattened pieces of the reserved cookie dough.

Bake for 24-26 minutes. The fudge layer should be crinkly on top and the cookie dough slightly golden brown. Don't over bake them or the bars will be hard and crispy.

Let the bars cool completely to room temperature before slicing into 12 rectangular squares (four by three). For very clean cuts, pop the bars in the fridge for about one hour to chill before slicing with a clean, sharp knife. Don’t forget to clean the knife between each cut!