1. Preheat the oven to 350°F (175°C). Prepare three 8-inch round cake pans with butter, parchment, and a dusting of flour.
2. Whisk flour, baking powder, baking soda, salt, and cinnamon in a medium bowl.
3. In a large bowl, cream butter, oil, and brown sugar until smooth and fluffy.
4. Add eggs one at a time, beating well after each addition. Stir in maple syrup and vanilla.
5. Add half of the dry ingredients, mix until just combined, then add sour cream and milk. Stir in the rest of the dry ingredients until just mixed.
6. Divide the batter among prepared pans. Bake for 20-25 minutes or until a toothpick comes out with moist crumbs.
7. Cool cakes in pans for 10 minutes, then transfer to wire racks to cool completely.

For the frosting:
8. Beat butter until creamy. Gradually mix in powdered sugar, salt, and vanilla on low.
9. Once combined, whip on high for 1 minute, then add maple syrup and whip again until fluffy.
10. Adjust with milk if needed for consistency and chill if too soft.
11. Assemble cooled cake layers with frosting and decorate with nuts if desired.