Preheat oven to 350 degrees. Grease and line a 13x9 dish with a parchment or foil sling.
To make the crust, beat butter until light and fluffy, about three to five minutes. Add in sugar, flour and salt and mix until mixture becomes crumbly and sticks together in clumps.
Press into lined pan and bake at 350 for 12-15 minutes or until golden brown at the edges.
To make the fudgy middle, combined sweetened condensed milk and chopped unsweetened chocolate in a medium saucepan. Cook over low heat until chocolate has melted and has combined with the sweetened condensed milk.
Pour over baked crust and gently spread to the edges. Set aside.
To make the topping, cream together 1/2 cup unsalted butter, 3/4 cup flour, 1/4 cup sugar, and 1/4 teaspoon kosher salt. Mix together until crumbly, then mix in chocolate chips and nuts.
Sprinkle over the top of the fudgy layer, and bake for an additional 30-35 minutes.
Cool completely on wire rack before removing from pan and cutting into squares.