Preparation

Prepare the beets

• Position a rack in the center of the oven and heat the oven to 375°F. Put the beets on a piece of foil large enough to wrap them. Drizzle with the vegetable oil and turn to coat well. Enclose the beets in the foil and roast until tender when pierced with a paring knife, about 1 hour. Let cool.

Make the cake

• Reduce the oven temperature to 350°F. Coat two 9-inch round cake pans generously with softened butter. Line the bottom of the pans with parchment and coat the parchment with butter. Dust the pans with flour, tapping out any excess.
• Peel and finely grate enough of the beets to yield 3/4 cup. Sift the flour, cocoa powder, baking soda, and salt into a medium bowl.
• Using a stand mixer fitted with the paddle attachment or an electric hand mixer, beat the butter and sugar on medium-low speed until fluffy, about 3 minutes. Beat in the eggs, one at a time. Mix in the grated beets and the vanilla. Reduce the speed to low and carefully add half of the flour mixture; mix until fully incorporated. Add 1-1/4 cups hot water and the remaining flour mixture, return to medium-low speed and mix until smooth, about 2 minutes. Divide the batter evenly between the prepared pans, smoothing the tops.
• 烤蛋糕,旋转中途烘焙,until a toothpick inserted in the center of the cakes comes out clean, about 25 minutes. Cool the cakes in their pans on a rack for 10 minutes and then turn them out onto racks and peel off the parchment. Let the cakes cool completely.

Make the frosting

• Melt the butter in a 3-quart saucepan over medium heat. Add the sugar and cocoa powder and mix until combined. Stir in the cream, espresso powder, vanilla, and salt. Bring the mixture to a simmer and cook, stirring constantly, until smooth. Pour the mixture into a bowl and cool slightly. Refrigerate, stirring every 10 minutes, until soft peaks form and the frosting is completely cool, about 1 hour.

Assemble the cake

• Place one of the cakes on a cake plate and spread a generous 1/2 cup frosting evenly over the top. Top with the second cake and spread a generous 1/2 cup frosting over it. Frost the sides with the remaining frosting.

Make Ahead Tips

The beets can be roasted up to 3 days ahead and refrigerated. The cake can be assembled up to 2 days ahead. Cover with a cake dome and store at room temperature.