For the cake:
3 tablespoons natural unsweetened cocoa powder, sifted
1/3 cup boiling water
1 cup buttermilk
3/4 cup (1 1/2 sticks) unsalted butter, at room temperature
1 1/2 cups granulated sugar
3 large eggs
2 teaspoons red gel food coloring
2 teaspoons pure vanilla extract
1/2 teaspoon fine salt
2 1/2 cups all-purpose flour
1 1/2 teaspoons baking soda
1 teaspoon distilled white vinegar
For the frosting:
1 pound cream cheese, at room temperature
1/2 cup (1 stick) unsalted butter, at room temperature
1 tablespoon pure vanilla extract
2 cups powdered sugar, sifted
Pinch of fine salt
1/2 cup chopped toasted pecans (optional)
Heat the oven to 350°F and arrange a rack in the middle. Generously coat a 10-cup Bundt pan with cooking spray.
To make the cake, sift the cocoa into a bowl and pour the boiling water over it. Whisk to combine, then whisk in the buttermilk. In the bowl of a stand mixer fitted with the paddle attachment, beat the butter and sugar on medium-high speed until creamy, about 3 minutes. Add the eggs 1 at a time, beating well after each addition. Stir in the food coloring, vanilla, and salt.
Reduce the mixer’s speed to medium-low and carefully add a third of the flour, followed by half the buttermilk. Repeat, then turn off the mixer and scrape down the sides. Add the final third of flour and mix until just combined.
In a small bowl, stir together the baking soda and vinegar, then quickly stir this mixture into the batter.
Scrape the batter into the prepared Bundt pan and smooth the top.Bake until puffed and a toothpick inserted into the center comes out clean, about 40 minutes. Cool the cake in the pan on a wire rack for about 15 minutes, then unmold the cake directly onto the rack and cool completely.
Meanwhile, make the frosting. In the bowl of a stand mixer fitted with the paddle attachment, beat the cream cheese, butter, and vanilla on medium-high speed until smooth and fluffy, about 3 minutes. Gradually beat in the powdered sugar and salt and continue to mix until thoroughly combined, scraping down the sides of the bowl as needed.
To finish, slide the cake onto a serving platter. Spread the top (and sides, if you like) with the frosting and sprinkle with the pecans, if using.