First, preheat the oven to 350 degrees Fahrenheit or 175 degrees celsius.
Take a 9x13 inch metal baking pan and grease some butter, so the mallowmelt doesn't stick.
Take a small microwave-safe bowl and fill it with 1 cup butterscotch chips and half a cup butter. Microwave for 20 seconds. Stir them well now. If the butter remains in solid form, put the bowl back in the micro-wave for 20 seconds.
Stir the butter and butterscotch chips together until they're completely combined. Make sure you don't overheat the butter. If there are a few lumps, it's fine. Now, refrigerate it to cool properly.
In a large mixing bowl, add 2/3 cup brown sugar, 2 teaspoons vanilla essence, and 2 large eggs. Stir until combined and the sugar has dissolved.
In a medium mixing bowl, combine 1.5 cups flour with 2 teaspoons baking powder and half teaspoon salt. Stir until everything is well incorporated.
The butterscotch mixture should have cooled by now. Remove the bowl from the refrigerator and add it to the large mixing bowl containing brown sugar, vanilla extract, and egg. Stir until completely combined.
Next, pour the flour mixture into the butterscotch and brown sugar mixture you've just combined. Stir everything until completely mixed.
Now, add 2 cups of marshmallows, 1 cup of chocolate chips and 1/2 cup of the butterscotch chips in the same bowl. Combine them well.
After that, pour the batter into the prepared baking pan. Spread them evenly into the pan. Then, scatter the remaining marshmallows, chocolate, and butterscotch chips over them for decorations. Finally, drizzle everything with a can of sweetened and condensed milk to finish it off.
Now, set it in the preheated oven and bake for 25-30 minutes or until firm. The top will be golden brown, yet it will still be gooey.
Allow the mallowmelt to thoroughly cool before serving. You should allow at least 2-3 hours. Then, using a knife, cut the dish into slices and serve the children and yourself!