Preheat the oven to 350° F and place a rack in the center of the oven.
Lightly grease a 9"x13" baking pan and set aside.
In a medium bowl, combine flour, brown sugar, granulated white sugar, and cinnamon; then add oil.
In a separate bowl take 3/4 cup of the flour mixture and add chopped nuts, set aside (for streusel topping).
Next, stir in baking powder and baking soda.
Mix in buttermilk and egg until just well incorporated.
Pour batter into baking pan and sprinkle with reserved streusel topping.
Bake for 35 minutes, or until a toothpick inserted in the center comes out clean.
Set aside for the cake to cool down for 5-10 minutes.
Cut, serve and enjoy!!!