To make the cake, preheat oven to 350 degrees F. Butter 15x10-inch jelly roll pan and line with parchment paper.
Place eggs in large bowl; beat using electric mixer on high speed 5 minutes. The whipped eggs will become thick and lightened in color. With the mixer still running, slowly add sugar and oil, followed by buttermilk and vanilla.
In a separate bowl, sift together flour, cocoa, baking powder and salt. Slowly add to the liquid ingredients. Mix until well combined. Pour batter into prepared pan and smooth evenly with a rubber spatula. Bake 12-15 minutes. Cake is done when it springs back when pressed with fingers.
Sprinkle a tea towel with powdered sugar and rub sugar into towel with your hands. Immediately turn cake out onto the sugar-covered tea towel. Peel off the parchment paper and roll cake into the tea towel, beginning at the narrow end. Place rolled cake on a wire rack, seam-side down, and let cool completely while you make the pumpkin buttercream.
To make the buttercream, place pumpkin puree and butter in large bowl and whip together using an electric mixer on high speed. Gradually add in powdered sugar a little at a time. Add just enough to make a thick buttercream of piping consistency; you may not have to use all of the powdered sugar. Add a few drops of orange gel food color if desired. Finally, add pumpkin pie spice and whip again on high speed. Transfer buttercream to a piping bag with 1/2-inch hole cut in the end of the bag.
To assemble cake, gently unroll the cake and pipe buttercream over top of cake. Roll the cake back up and wrap in plastic wrap. Refrigerate until the frosting is firm, about 1 hour.
Combine semisweet chocolate and heavy cream in a microwave-safe bowl and heat at 100% power, at 30 second intervals until the mixture is hot but not boiling (about 1 minute total). Let the bowl of cream and chocolate stand for one minute before whisking together. Place the chilled cake on a wire rack and place the rack on a baking sheet larger than the rack. Pour the ganache over the cake until it is completely covered. To garnish, dollop or pipe whipped cream on top of the cake and add chocolate sprinkles. Place the two wafer cookies, crossed, on top of the cake as a final flourish.