Preheat oven to 350 degrees. Butter and flour eight 8-oz. bundt pans. Combine all-purpose and whole wheat flours, baking powder, baking soda, salt, and spices in a large bowl. Stir with a whisk to blend. In a separate bowl, beat butter and sugar using an electric mixer at medium speed until fluffy. Add eggs, one at a time. Add espresso coffee and maple syrup and blend thoroughly. At low speed, beat in flour mixture and sour cream alternately. Spoon equal amounts of batter into prepared bundt pans and bake in oven for 30 to 35 minutes or until a wooden pick inserted into the cakes comes out clean. Cool on a wire rack for 5 minutes, then invert cakes out of the pans and continue to cool. To make sauce, combine maple syrup and butter in a small saucepan and simmer for 15 minutes. Remove from heat and stir in the espresso and bourbon. Brush each cake with some of the sauce. Add walnuts to the remaining sauce. When ready to serve, reheat sauce just until warm and pour a spoonful over each bundt cake. Serve with a small scoop of vanilla ice cream, if desired. Makes 8 servings.

Note: One 8- to 9-inch bundt pan can be used instead of individual 8-oz. pans. Bake the large bundt cake for 45 to 50 minutes or until a wooden pick inserted into cake comes out clean.