Preheat oven to (or 350°F).
Melt the butter and let it cool slightly.
Pour the butter in a big bowl. Add the vegetable oil, the eggs, the vanilla extract, and all the sugar.
Mix well until everything is foamy.
Add the sweet potato puree, the flour, the baking powder, ground cinnamon, nutmeg and salt. Mix well.
Divide the dough evenly among the cake pans.
Bake for ca. 25 minutes.
Let the cakes cool in their pans for 10 minutes before you get them out.
For the frosting, bring the maple sirup to a boil in a little pot.
Simmer for about 10 minutes.
Add the eggs to a bowl.
Beat the eggs until they are thick and foamy.
Slowly drizzle the hot maple sirup into the bowl with the eggs while you continue to beat everything at high speed.
Beat until the mix has reached room temperature.
Add cubes of butter and continue to beat.
Add the powdered sugar and beat everything until smooth and creamy.
Put one cake layer on a plate. Spread a thin layer of buttercream on top. Add about half of the pecan pieces. Spread another thin layer of buttercream on top.
Put the second cake layer on the buttercream. Now spread the remaining buttercream on the top and side of the whole cake.
Decorate with the remaining pecan pieces.