1. Position oven rack to lowest third of oven. Heat oven to 350 degree F. Line bottoms of two 8-inch round cake pans with parchment paper. Grease and flour parchment and sides of pans. Set aside.
2. Sift the flour, baking powder, baking soda, and salt in a medium bowl and set aside.
3. Combine buttermilk and vanilla in a cup. In a mixer bowl on medium speed cream the butter until satiny. Add sugar in a steady stream, then scrape down the sides of the bowl. Continue beating until light and fluffy, 4 to 5 minutes. Slowly add eggs and beat until mixture appears fluffy, white and increased in volume, 3 to 4 minutes. Stop mixing and scrape sides of the bowl.
4. By hand, stir 1/4 of flour mixture into creamed mixture. Then blend in 1/3 of milk mixture. Repeat, alternating dry and liquid ingredients, ending with the flour and mixing until smooth. Spoon batter equally into each pan. Spread the batter, working from the center outward, creating a slightly raised ridge along the outside rim so the baked cakes will end up being level. Bake 30 to 35 minutes, or until the sides begin to pull away from the pans. Cool on wire racks 10 minutes. Invert cakes onto wire racks and carefully remove pans and parchment. Invert the layers right side up and cool completely.Make maple meringue::
5. In a heavy-bottomed saucepan over medium heat, bring maple syrup to a boil. As syrup boils, whisk egg whites in a mixer bowl on low speed until small bubbles appear, about 30 seconds. Increase speed to medium, add the sugar and beat until stiff peaks form, 2 minutes. With the mixer still on, slowly pour hot syrup in a steady stream directly down the side of the bowl. Whip 2 minutes, then reduce speed to low and continue whipping as meringue cools to room temperature and thickens, 5 to 7 minutes.
6. To assemble, split each cake layer in half. Place one layer on a cake plate and spread with maple meringue about 1/4 inch thick. Top with another cake layer, gently pressing it to level the cake. Spread with more meringue. Repeat with the third layer, leveling it, then spreading with meringue. Top with the last layer, again pressing it carefully to level. Ice the top and sides of the cake with remaining meringue, and decorate with walnuts, if desired. Makes 8 to 10 servings.