Make the brownie batter:

Preheat the oven to 350ºF.

Line an 8x8-inch baking pan with parchment paper and grease it with cooking spray or butter.

In a large bowl, combine the melted butter with the sugar, cocoa powder, vanilla extract and salt, stirring to combine.

Whisk together the eggs in a small bowl then stir them into the brownie batter. Fold in the flour, just until combined, then pour the batter into the prepared pan, reserving 1/4 cup of the batter.

Make the butterscotch cheesecake:

Melt the butterscotch chips in a microwave-safe bowl on 20-second intervals, stirring between each interval. Set the melted butterscotch aside to cool slightly for about 3 minutes.

In the bowl of a stand mixer fitted with the paddle attachment, beat together the cream cheese and egg yolk until well combined. Beat in the melted butterscotch. Add dollops of the the cheesecake mixture and reserved brownie batter alternately on top of the brownie batter in the pan. Using a knife or toothpick, swirl the cheesecake mixture and brownie batter together.

Bake the brownies for about 30 minutes, or until a toothpick inserted comes out with a few crumbs. Remove the brownies from the oven and let them cool completely before lifting the parchment paper out of the pan. Cut the brownies into 12 or 16 squares and serve.