Prepare the cake: Preheat oven to 350F. Lightly grease a 9x13 baking pan. Set aside.
In a large bowl, whisk pumpkin, xylitol (or sugar) oil, sour cream, eggs, and vanilla until smooth. In a separate bowl, combine remaining cake ingredients. Add dry mixture to liquid mixture, gently folding until combined.
Spread batter into prepared pan, evenly smoothing top. Bake cake at 350F 30-35 minutes or until a toothpick comes out clean. Cool cake completely in pan on a wire cooling rack.
Prepare the frosting: In a medium bowl with an electric handheld mixer, beat frosting ingredients on high speed until smooth and creamy (if frosting seems to thick, add a little more cream)
Spread frosting evenly over cooled cake and sprinkle with nuts if desired. Refrigerate until ready to serve.