Servings: 1 (8-inch) two-layer cake, 8 to 12 slices

2 cups whole walnuts 2 cups whole walnuts
1/4 cup dark cocoa ¼ cup dark cocoa
1/4 cup boiling water ¼ cup boiling water
2 tablespoons red food coloring 2 tablespoons red food coloring
2 1/4 cups pastry flour 2 ¼ cups pastry flour
1 teaspoon salt 1 teaspoon salt
1/2 cup butter, room temperature ½ cup butter, room temperature
1 1/2 cups granulated sugar 1 ½ cups granulated sugar
2 large eggs 2 large eggs
1 teaspoon pure vanilla 1 teaspoon pure vanilla
1 cup buttermilk 1 cup buttermilk
1 teaspoon baking soda 1 teaspoon baking soda
1 tablespoon white vinegar 1 tablespoon white vinegar

Preheat the oven to 350°F.
Spread the walnuts across the prepared tray and bake for 10 minutes, or until nicely golden. You can use a metal spatula to give them a turn at the halfway mark so they toast evenly. Remove them from the oven and allow them to cool before roughly chopping.
In a small bowl, combine the cocoa, boiling water, and red food coloring. Whisk to combine, being careful, as you don't want it splashing all over your counter! Set aside.
On a large piece of parchment paper, sift together the flour and salt. Set aside.
In a stand mixer fitted with a paddle attachment, cream the butter on medium speed until light and fluffy. Scrape down the sides of the bowl. Add the sugar, eggs, and vanilla and beat again until well combined. Scrape down the sides of the bowl.
Turn the mixer speed to low and add the cocoa mixture. Continue to beat until combined. Scrape down the sides of the bowl.
Add the flour and buttermilk alternately, beginning and ending with the flour. Scrape down the sides of the bowl. Add the baking soda and then the vinegar and beat again.
Divide the batter evenly between the two prepared pans.
Bake for 25 to 30 minutes, or until a wooden skewer inserted in the center of a cake comes out clean.
Remove the cakes from the oven, allow them to cool in the pans for about 10 minutes and then invert them onto wire racks to cool completely. You may need to run a sharp knife around the edges of the pans if the cakes do not fall when first inverted.
Meanwhile, prepare cream cheese buttercream.
Transfer your first cake layer to a cake stand. Use an offset spatula to spread the buttercream generously across the layer. Top with your second layer and continue to cover the top and sides with a nice smooth layer of buttercream. Don't worry too much about perfection as the toasted walnuts will hide a lot of flaws!
Once the cake is covered in buttercream, scoop up large handfuls of the chopped toasted walnuts and press them into the sides of the cake. It helps to place your cake stand on a cookie sheet to catch any falling walnuts, which you can then scoop up and use again.