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Heat the oven to 350°F. Generously spray a 10- to 12-cup tube pan (the kind without a removable bottom) with baking spray. Rub all over with a paper towel, then spray lightly one more time.
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In a medium bowl, combine the flour, baking powder, ginger, cinnamon, salt, baking soda, nutmeg, and cloves. In a measuring cup, combine the buttermilk, bourbon, and vanilla.
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In a stand mixer fitted with the paddle attachment, beat the butter on medium speed for 5 minutes. Slowly and gradually add the oil, beating until combined. Add the brown sugar. Increase the mixer speed to high and beat, scraping down the sides of the bowl as needed, until light and fluffy, about 4 minutes. Add the eggs one at a time, beating well on medium-high speed after each addition. Beat in the pumpkin.
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Slowly pour in the buttermilk mixture, and mix until combined. Gradually add the dry ingredients to the batter, mixing just until combined. Scrape the batter into the prepared pan, smooth the top, and tap the pan on the counter once or twice to settle the batter.
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Bake until a wooden skewer inserted into the center of the cake comes out with just a few moist crumbs clinging to it, 45 to 60 minutes. Cool the cake in the pan for 10 to 15 minutes, invert the cake onto a rack, removing the pan, and cool completely, at least 2 hours.
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For the icing, combine the sugar, butter, milk and salt in a medium saucepan over medium-high heat. Bring to a boil, stirring constantly, and cook for 3 minutes.
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Remove from the heat and add vanilla extract. (At this point, the icing will be the consistency of thick syrup.) Use an electric mixer on medium speed and beat for about 1 minute.
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Add powdered sugar and beat again till smooth. If needed, add more milk to make an icing that can easily be drizzled.
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When cake is cooled, slowly spoon icing over cake and allow to drip down the sides, pooling a bit on your cake plate.
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Top with walnuts, if desired, before icing hardens.