Preheat the oven to 350°F. Grease a 9" cake pan and set it aside. In a small bowl, make the streusel by stirring the melted butter, flour, brown sugar, cinnamon and salt together with a fork until it is combined and crumbly. Then stir in the chopped nuts. Set this mixture aside.

For the cake batter, start by whisking together the melted butter, both sugars and eggs until combined. Then fold in the carrots. Next add in both of the flours, baking soda, cinnamon and salt. Fold the batter together until it is just combined.