Heat the oven to 350°F. Line a standard 12-cup muffin tin with paper liners. Combine the flour, cinnamon, baking powder, and salt in a medium bowl.
Combine the butter and sugar until fluffy in a large bowl and beat with an electric mixer until fluffy, about 4 minutes. Add the eggs and egg yolks, one at a time, then the vanilla. Add the flour mixture and the buttermilk into the sugar and egg mixture in increments, starting and ending with the flour mixture. Fold in the walnuts.
Divide the batter between the liners. Bake until a toothpick inserted into the cupcakes comes out with just a few moist crumbs, 22 to 25 minutes. Transfer cupcakes to a rack to cool completely.
Prepare the frosting: Beat the butter and confectioners’ sugar in a large bowl until light and fluffy. Beat in the maple syrup and salt. Top the cupcakes with the frosting and chopped walnuts, if desired.