Heat oven to 350°F (175°C) and put butter on a 9x13-inch baking pan.
In a safe microwave bowl, mix 1 cup butterscotch chips and 1/2 cup butter. Heat for 20 seconds, stir, and repeat if needed. Put it in the fridge to cool.
In a big bowl, mix 2/3 cup brown sugar, 2 tsp vanilla, and 2 eggs until the sugar melts.
In a medium bowl, mix 1.5 cups flour, 2 tsp baking powder, and 1/2 tsp salt.
Put the cooled butterscotch mix into the big bowl and mix.
Put the flour mix into the butterscotch mix and stir.
Add 2 cups marshmallows, 1 cup chocolate chips, and 1/2 cup butterscotch chips, and mix well.
Pour everything into the pan and put the rest of the marshmallows, chocolate, and butterscotch chips on top. Pour sweetened condensed milk on it.
Bake for 25-30 minutes until it's firm and golden on top but still gooey inside.
Let it cool for 2-3 hours, then cut and serve.