Preheat oven to 350 degrees F. Line a 9X13-inch pan with aluminum foil or parchment paper, and lightly spray with nonstick cooking spray.
In a large bowl, whisk together the flour, salt and brown sugar. Add the butter and eggs and mix with an electric mixer until crumbly. Press the dough together with your hands. It will have a texture similar to shortbread.
Mix in 1 1/2 cups of the chocolate chips and the nuts (if using). The mixture will be fairly stiff, so it works best to mix with your hands.
Reserve 1 1/2 cups of the dough and set aside.
Press the remaining dough into the bottom of the prepared pan. Bake for 10 minutes until just starting to set (should still be somewhat soft).
Pour the sweetened condensed milk evenly over the baked crust. Top with all but 1/4 cup of the toffee bits.
Pinch pieces of the reserved dough and scatter over the top. Sprinkle with remaining 1/2 cup chocolate chips. Bake for 25-30 more minutes until golden brown. Sprinkle the remaining 1/4 cup toffee bits on top. Cool completely before serving.