Position a rack in the lower third of the oven and preheat to 350 degrees F.
Line a large (4 1/2-inch by 9 1/2-inch) metal loaf pan with parchment paper and spray with nonstick cooking spray.
Sift the flour, baking soda, ginger and salt into a medium bowl. Whisk the pumpkin, buttermilk, and vanilla in another bowl.
Beat the butter with an electric mixer at medium speed until smooth, about 1 minute. Add the light brown sugar and continue beating until light and fluffy, about 4 minutes more. Add the eggs, 1 at a time, beating well after each addition. At low speed, add the flour mixture in 3 parts, alternating with the pumpkin mixture in 2 parts, beginning and ending with the flour. Mix briefly at medium-low speed to make a smooth batter. Turn off beaters and add the crystallized ginger. Finish folding batter--which will be thick--by hand with a rubber spatula.
Scrape batter into the prepared pan and smooth the top. Bake until a cake tester inserted into the cake comes out clean and top springs back when lightly pressed, about 55 minutes. Cool in pan on a rack 10 to 15 minutes, then invert pan to release cake onto rack to cool. Dust with confectioners' sugar, if desired. Serve.
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