This recipe is excerpted from Alice Medrich's Flavor Flours (Artisan).
Position a rack in the lower third of the oven and preheat the oven to 350 degrees F. Line the bottom and all four sides of the loaf pan with parchment paper.
Combine the butter, sugar, and eggs in the bowl of the stand mixer and beat on medium speed with the paddle attachment until lighter in color, about 2 minutes. Or beat with the handheld mixer in a large bowl on medium-high speed for 3 to 4 minutes.
Add the rice and buckwheat flours, baking soda, baking powder, cinnamon, nutmeg, salt, pumpkin puree, and raisins and beat on low speed until smooth. Scrape the mixture into the prepared pan.
Bake the loaf for 45 to 50 minutes, until a toothpick inserted in the center comes out clean. Cool the loaf in the pan on a rack for at least 2 hours before unmolding and slicing. The cake keeps, wrapped airtight, in the refrigerator for up to 5 days; let come to room temperature before serving.