Preheat oven to 325°. Lightly coat an 8-inch baking pan with cooking spray.
Melt butter with chocolate in a medium saucepan over low heat. Remove pan from heat, and stir in sour cream, sugars, eggs, and vanilla until well-combined. Stir in flour, salt, and chocolate chips.
Pour batter into prepared pan; bake in middle of oven until a toothpick comes out clean (about 35 minutes).
Cool brownies in a pan; cut into 16 squares. Melt butterscotch chips in a small pan over low heat, stirring (about 4 minutes). Add milk, and whisk until smooth. Pour butterscotch into a ziptop plastic bag; snip end off one of the bottom corners. Stack brownies (2-4 in each stack); drizzle each stack with butterscotch.