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1
Heat oven to 350 degrees F. Butter a 9-inch round baking pan that is at least 3 inches tall. (Otherwise, use a larger pan or it may overflow.)
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2
Make ginger crumb topping: Mix sugar, flour, and candied ginger together in a bowl, then stir in melted butter. Put the topping in the freezer while you mix up the cake.
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3
Make the cake: whisk flour, baking powder, ginger, baking soda, and salt in a bowl. Using a handheld mixer with beaters, or a stand mixer with paddle attachment, cream the butter and sugar together on medium-high speed until light and fluffy, about 3 to 5 minutes. Add the eggs, one at a time, scraping down sides of bowl after each addition. Stir in the flour mixture in three additions, alternating with buttermilk in two additions, beginning and ending with dry ingredients, and scraping down the sides of the bowl occasionally. Gently fold in rhubarb.
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4
Spread the batter into the prepared pan, then sprinkle the crumb topping over the cake-make sure the crumb is totally frozen, and that you move quickly to sprinkle it over the cake and transfer to the oven. Otherwise it will melt into the cake, which will still taste good but won't look as spectacular. Bake for 45 to 50 minutes, or until lightly golden, and firm on top.
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5
Store wrapped in plastic, at room temperature, for 2 to 3 days.
