Preheat the oven 350°F. Prepare your baking pan by coating with cooking spray or butter.
In a medium bowl, whisk together the sugar, flour, baking soda, cinnamon and salt. In a large mixing bowl, beat the eggs until light and frothy. Beat in the oil. Add the flour mixture in two additions, until just incorporated, scraping down the sides of the bowl. Stir in the pumpkin and ground almonds.
Pour the batter into the pan and place in the oven. Bake for 30 minutes, then check. If using a 13"x9" pan, bake up to 45-50 minutes total time, or until the cake is springy and a toothpick inserted in the center comes clean. If using a springform pan, lower the temperature to 325°F and bake up to 50 minutes total time, or until a toothpick inserted in the center comes clean.
When the cake is baked, remove from oven. Let it cool completely. If using a springform pan, remove sides and trim the top of the cake with a serrated knife for a flat surface. Invert onto your wire rack or serving plate. Remove bottom of springform pan if using. Wrap the cake well and refrigerate until ready to frost. This can be done a day or two in advance.
Make the frosting by whipping together the butter and mascarpone cheese. Add the vanilla and cinnamon. Slowly add the powdered sugar and whisk well. Frost the cake and refrigerate.
Remove the cake from the refrigerator about 10-15 minutes prior to serving.
What size pan to use? This recipe is written for baking in one 9" round pan, but can also be baked in a 13"x9" rectangular pan or even a springform pan. The recipe instructions provide additional details on adjusting baking times to suit the pan size you've chosen.