Get Ready: Preheat your oven to 350°F (175°C). Lightly grease a 9x13 inch baking dish or line it with parchment paper for easy removal.
Make the Crumb Base: In a large bowl, beat the softened butter and brown sugar together until light and fluffy (about 2-3 minutes). Stir in the flour and salt until the mixture is crumbly.
Form the Crust: Take about ⅔ cup of this crumb mixture and set it aside – this will be your topping! Press the remaining crumb mixture evenly into the bottom of your prepared baking dish to form the crust.
Bake the Crust: Pop the crust into the oven for 10 minutes, or until it's just starting to turn golden at the edges.
Prepare the Crumb Topping: Stir the chopped pecans (if using) into the crumb mixture you set aside. This is now your crunchy crumb topping!
Melt the Chocolate Layer: In a microwave-safe bowl, combine the sweetened condensed milk and bittersweet chocolate chips. Heat in 30-second bursts, stirring well after each, until the chocolate is smoothly melted and combined with the milk.
Layer It Up: Pour the melted chocolate mixture evenly over the hot, pre-baked crust. Spread it with a spatula.
Add the Goodies: Sprinkle your pecan-crumb topping over the chocolate layer. Dot spoonfuls of raspberry preserves evenly over the crumb topping (if preserves are too thick, stir in a tablespoon of water to loosen). Finish by sprinkling the semi-sweet chocolate chips all over the top.
Bake to Perfection: Bake for another 25 minutes, or until the bars are set and the topping is lightly golden.
Cool and Enjoy: Let the bars cool completely before cutting them into squares. This step is important for neat slices!