Step 1Heat the oven to 350 degrees. Grease and flour a 9-inch round cake pan.
Step 2Combine the flour, brown sugar, granulated sugar, cinnamon and oil in a large bowl, mixing well. Remove 3/4 cup and set aside.
Step 3Whisk together the baking powder, baking soda, egg and buttermilk in a small bowl. Let stand 5 minutes. The mixture should about double in size and be a bit bubbly. Blend it into the flour mixture. Pour the batter into the cake pan.
Step 4Mix the walnuts with the reserved 3/4 cup topping and sprinkle over the top. Bake the cake for 10 minutes, then reduce the heat to 325 degrees and bake until a toothpick inserted in the middle comes out clean, 45 minutes.
Step 5Cool the cake on a wire rack 10 minutes before cutting. Serve warm.