To make the cookie dough layer:

Line a 9×9” or 8×8″ metal pan with parchment overhang on all sides (so crossed over each other). Beat together the butter, sugar, vanilla and salt until light and fluffy. Add the flour and sweetened condensed milk and beat to combine. Fold in the chocolate chips. Plop the dough onto the prepared pan and spread it around to make one flat layer. Freeze the layer for a few hours, or overnight.

To make the brownie batter:

Preheat the oven to 340 F. Over a bain marie (a heatproof bowl over simmering water, where the water does not touch the bottom of the bowl) add the butter. Once it has melted add the sugar and cocoa. Stir it until the temperature reaches 140 F. Remove from heat and add the salt and vanilla and whisk. Once no longer too hot to touch, add the eggs and whisk the batter for a few minutes until you have a very shiny, smooth batter. Fold in the flour and the chocolate.

To make the cookie dough brownies:

Using the parchment overhang, remove the frozen cookie dough layer. Return the parchment to the pan and pour in ⅔ of the batter. Set the frozen square over the batter and spread the remaining ⅓ brownie batter over it. Bake until the middle has slightly puffed to show doneness, about 45 minutes.