Begin by preheating your oven to 350°F (175°C).
Grease a baking pan with butter to prevent sticking.
In a microwave-safe bowl, melt 1 cup butterscotch chips and 1/2 cup butter. Stir. Chill the mixture briefly for proper cooling.
In a bigger bowl, mix 2/3 cup brown sugar, 2 tsp vanilla, and 2 eggs.
Combine 1.5 cups flour, 2 tsp baking powder, 1/2 tsp salt in another bowl.
Add cooled butterscotch mix to the sugar mixture. Mix well.
Combine the flour mix with the butterscotch mix. Stir until blended.
Add marshmallows, chocolate chips, and butterscotch chips. Mix.
Pour batter into the pan, add toppings, and drizzle condensed milk.
Bake 20-25 minutes at 350°F (175°C) until set, golden, gooey.
Cool completely (1-2 hours) before slicing and serving. Enjoy!