Preheat your oven to 350°F (175°C). Lightly grease a 9x13 inch baking dish or line it with parchment paper for easy removal.
In a large mixing bowl, beat together the softened butter and brown sugar for about 2-3 minutes until the mixture is light and creamy.
Add the flour and salt, mixing just until everything is combined and the dough looks crumbly.
Set aside about ⅔ cup of this delicious crumbly mixture; this will become your topping! Press the remaining dough evenly into the bottom of your prepared baking dish to form the crust.
Pop the crust into the preheated oven and bake for 10 minutes, or until it just starts to turn a lovely golden brown.
While the crust bakes, stir the chopped pecans (or walnuts) into the reserved crumb mixture. This is your irresistible nutty crumble topping!
In a microwave-safe bowl, combine the sweetened condensed milk and bittersweet chocolate chips. Heat this mixture in 30-second intervals in the microwave, stirring well after each burst, until the chocolate is fully melted and the mixture is wonderfully smooth. (You can also use a double boiler on the stove for a gentle melt.)
Once the crust is out of the oven, pour the warm chocolate mixture evenly over it, spreading it with an offset spatula.
Generously sprinkle your nutty crumb topping over the chocolate layer.
Now, carefully drop small teaspoonfuls of raspberry preserves over the crumb topping. (If your preserves seem too thick, stir in a tablespoon of water to loosen them up a bit.)
Finish by sprinkling the semi-sweet chocolate chips over everything.
Bake for another 25-30 minutes, or until the bars are nicely set.
The hardest part: let the bars cool completely before slicing them into squares. This ensures perfect, neat bars! Then, enjoy your homemade treat!