Adjust an oven rack to the middle position. Preheat the oven to 350°F. Grease a 9 x13-inch pan and line it with a parchment sling.
Melt the chocolate by microwaving it in 3 intervals of 30 seconds each (with a stir in
between), until melted [or use a double boiler].
In a large bowl, whisk the sweetened condensed milk, chocolate, heavy cream, vanilla, and salt until smooth.
In the bowl of a stand mixer fitted with a paddle, mix the flour, oats, granulated and brown sugars, baking soda, and salt on low to combine. Add the butter and mix on medium until the mixture is crumbly.
Press about ⅔ of the oat mixture into the bottom of the prepared pan. Bake 10 minutes. Remove the pan from the oven and carefully spread the chocolate mixture over the crust. Sprinkle the remaining ⅓ oat mixture evenly over the top. Bake 15 to 20 minutes, until the chocolate has puffed up a bit and does not jiggle, and the crumbly top is light golden brown.
Transfer the pan to a wire rack and let cool. Place the pan in the fridge and chill for 4 to 6 hours.
Slice the bars and serve.