Preheat the oven to 350°F. Line an 8-inch square baking pan with parchment paper. In the bowl of a stand mixer beat together the melted butter dark brown and granulated sugars until smooth. Add the eggs and vanilla mixing until well combined. With the mixer on the lowest speed add flour and salt and mix until just combined. Stir in a ½ cup of the toffee bits and ½ cup chocolate chips. Pour the batter into the prepared pan and smooth the top. Sprinkle the remaining ¼ cup of toffee bits and ¼ cup chocolate chips over the top of the batter. Bake for 35 to 40 minutes until a toothpick inserted comes out clean. Let the pan cool completely then remove the blondies from the pan. Cut them into squares and serve.