This towering cake is a real old-fashioned treat, as delicious as it is charming. Sour cream and toasted walnuts give it a rich, nutty flavor; the maple syrup gives it a warm, sweet finish.
Preheat your oven to 350°. Butter and flour three 8-inch cake pans.
First, prepare the walnuts: In a medium-size skillet over medium heat, toast nuts lightly until light brown and fragrant, 5 to 7 minutes. Cool nuts; then pulse in a blender or food processor until nuts have the texture of bulgur. Set aside.
Next, make the cake: In a large bowl, cream butter and sugars with an electric mixer until light and fluffy. In another bowl, beat eggs and yolks lightly with vanilla; then add to butter mixture and beat until incorporated. In a separate bowl, whisk together flour, baking powder, salt, and baking soda; set aside. In a fourth bowl, whisk together maple syrup, milk, and sour cream; set aside.
Add one-third of the dry ingredients to the butter/egg mixture and stir until just moistened; then add one-third of the maple/sour-cream mixture. Repeat until all ingredients are combined. Gently fold in walnuts. Divide batter among the three prepared cake pans. Bake 30 minutes, or until cake springs back to the touch.
Meanwhile, make the frosting: Put syrup in a small saucepan over medium-high heat and simmer, uncovered, until reduced to 2 tablespoons, 15 to 20 minutes. Set aside to cool. Cream together butter and cream cheese; then add sugar and mix well. Add cooled reduced syrup, cream, and vanilla; beat until smooth.
When cakes are done, cool them in their pans 30 to 45 minutes. Then remove from pans, layer, and frost. Garnish with walnut halves.