Preheat the oven to 180 degrees Celsius/ 350 degrees Fahrenheit. Grease and flour a loaf pan, about 30 cm/ 12 inches long.
Grate the pumpkin; no need to remove the peel if using Hokkaido. Then, mix the flour and the baking powder and sift them.
Beat egg whites: Divide the egg yolks from the egg whites. Whisk the egg whites until stiff, slowly add half of the sugar and continue whisking until the egg whites are glossy and stiff.
Make the batter: In another bowl, beat together the very soft butter and the remaining sugar until everything is pale and frothy, about 8 minutes. Incorporate the egg yolks and the vanilla extract shortly, then fold in the egg whites. Fold in the sifted flour, baking powder, pumpkin, and slivered almonds.
Bake: Pour the mixture into the prepared loaf pan and bake in the preheated oven, on the lowest part of the oven, for about 60-70 minutes, until a skewer inserted comes out moist but clean.
Cool: Leave the cake in the pan for about 15 minutes, then remove it and let it cool on a wire rack. Sprinkle with icing sugar before serving, if you like.