Preheat the oven to 350°F (175°C).
Grease and flour two 9-inch round cake pans.
In a large mixing bowl, combine the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
In another bowl, whisk together the buttermilk, oil, eggs, and vanilla extract until well combined.
Pour the wet ingredients into the dry ingredients and mix until just combined.
Fold in the chopped walnuts gently with a spatula.
Divide the batter evenly between the prepared pans, smoothing the tops.
Bake for 30-35 minutes, or until a toothpick inserted comes out clean.
Allow cakes to cool in pans for 10 minutes, then turn out onto wire racks to cool completely.
Once cool, spread a layer of peppermint frosting on top of one cake layer, then place the second layer on top.
Cover the entire cake with chocolate ganache for a glossy finish.
Finish with optional nut garnishes or additional frosting swirls for decoration.