For cake:

Put oven rack in center oven and preheat to 350 F.

Butter and flour two 9-inch cake pans, and line with buttered, floured parchment circles to fit bottoms.

In a medium bowl, whisk together eggs, egg yolks, buttermilk and vanilla.

Sift together flour, sugar, baking powder, and spices. Beat with electric mixer on slow for 30 seconds to combine evenly. Add butter and oil to dry ingredients and beat until incorporated, about 30 seconds more. Increase speed to medium and beat 1 1/2 minutes more, scraping down sides periodically. Add egg and buttermilk mixture in three even batches, beating about 30 seconds after each addition or until fully incorporated.

Divide batter between the two pans, smoothing the tops. Bake until tester comes out clean, 30-35 minutes. Cool 10 minutes, then invert on to cooling racks and remove parchment.

For frosting:

Whisk together mascarpone, brown sugar, pumpkin and spices until all blended. Then whisk in bourbon to taste.

In a separate bowl, whip cream until firm peaks form, then fold into pumpkin mixture. Refrigerate at least 15 minutes.

Measure out 2 1/4 cups of the frosting into a separate bowl. To this portion stir in 1 cup of the chopped pecans.

To assemble:

Halve the cake layers horizontally to form four layers. Place first cake layer cut side up as bottom of cake. Spread a heaping 3/4 cup of the frosting with nuts over the top. Place the next layer cut side down. Continue icing and stacking the layers. To top the final layer, spread with a portion of the nut-free frosting, making swirl patterns with a spatula, then ice the sides of the cake.

Sprinkle remaining pecans on the top of the cake in a ring on the outer edge.

Cover with cake dome and refrigerate at least 3 hours before serving.

Remove from refrigerator 15 minutes before serving.