Preheat oven to 300ºF. Rinse and wipe the sweet potatoes dry. Prick the surface with a fork at few places.
Wrap in aluminum foil and lay them on a baking sheet. Bake for 2 hours or more until a sharp knife passes through easily and it is cooked very soft - almost mushy.
When the sweet potatoes are completely cooled, peel and mash them well and set aside.
Grease a 10 inch round bundt cake pan and dust with flour all over. Set aside the prepared pan.
Grind the shelled walnuts with the light olive oil in a food processor or a high speed blender until very smooth to get the walnut butter.
To the walnut butter, add the brown butter and whisk it well. Add maple syrup, sugar, vanilla, salt and beat it until the mixture is fluffy.
Add eggs, one at a time and continue to beat. Stir the grated lemon zest into the mixture.
Sift the flour and baking powder into the mixture and stir it in. Mix the mashed sweet potatoes into the cake batter gently. The mixture should be of a slow dropping consistency.
Bake in a preheated oven for about 50 to 60 minutes until a cake tester or toothpick inserted comes out clean.
Remove the cake pan from the oven. Let it rest a few minutes and turn the bundt cake pan upside down to release the cake on to a serving platter.
Let the sweet potato cake cool before serving. Dust with confectioners sugar and grated citrus zest.
When oil replaces butter in a cake recipe, it is usually needed in a reduced amount than the butter.
When maple syrup/honey replaces sugar, it is usually needed in smaller quantity and adjustments are made to the amount of liquid added.
The baking temperature for the cake is lower when the batter is prepared with maple syrup or honey. But if only sugar is used, then the cake is to be baked at 350ºF.
Brown butter: Melt unsalted butter and allow it to simmer at a low setting. Allow the milk solids settled at the bottom of the pan to toast to a light brown. Strain the light golden liquid from the top and use in the recipe.