Preheat the oven to 350°F.
Position an oven rack in the center of the oven.
Lightly grease a 9x13-inch baking pan and set it aside.
In a medium mixing bowl, combine the all-purpose flour, brown sugar, granulated white sugar, and cinnamon.
Add the canola oil to the flour mixture.
Mix until the ingredients are fully combined and the mixture has a crumbly texture.
Transfer 3/4 cup of the flour mixture to a separate bowl.
Add the chopped walnuts to the reserved flour mixture.
Stir until the walnuts are evenly coated, then set the mixture aside for the topping.
Add the baking powder and baking soda to the remaining flour mixture.
Stir until the leavening ingredients are evenly distributed.
Add the buttermilk and egg to the flour mixture.
Mix until the batter is just combined and no large dry streaks remain.
Avoid overmixing, as this may affect the texture of the cake.
Pour the batter into the prepared baking pan.
Spread the batter evenly in the pan using a spatula.
Sprinkle the reserved walnut topping evenly over the surface of the batter.
Place the pan in the preheated oven.
Bake for approximately 35 minutes, or until a toothpick inserted into the center comes out clean.
Remove the cake from the oven.
Allow the cake to cool in the pan for 5 to 10 minutes before slicing.
Cut into slices and serve warm or at room temperature.