Preheat your oven to 350°F/175°C. Butter or grease an 8 inch square baking pan and line it with parchment paper.
In a large bowl, whisk together the sugar and egg for 1 minute.
175 grams granulated sugar, 1 egg
Add the buttermilk, hot coffee, oil, vanilla and salt. Whisk until it is smooth. Add your cocoa powder and whisk again until smooth
120 ml hot coffee, ¼ teaspoon vanilla extract, ½ teaspoon kosher salt, 120 ml buttermilk, 60 ml vegetable oil, 32 grams cocoa powder
Add the flour, baking powder, and baking soda and whisk until smooth. Fold in your chopped walnuts
110 grams all purpose flour, 1 teaspoon baking soda, ½ teaspoon baking powder, 64 grams walnuts
Pour into your pan and tap on the counter to get rid of any bubbles. Bake for 35-45 minutes. A toothpick inserted should come out clean. Allow it to cool for 20 minutes in the pan, then lift it out and let it cool completely on a cooling rack.
Make Frosting
Melt your chocolate in a microwave, stirring every 10 seconds so it doesn't burn.
85 grams dark chocolate
In your stand mixer with the paddle attachment, beat the butter and confectioners’ sugar on low speed until well combined
Turn the speed to high for 2-3 minutes until light and fluffy.
Add in your cocoa powder and beat on low, then increase the speed to medium for 1-2 minutes.
42 grams cocoa powder
Add your sour cream, melted chocolate and salt and mix once more.
pinch kosher salt, 60 ml sour cream
Spread over your cooled cake and decorate with walnuts.
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Notes
My recipes have been developed and tested in metric grams. I highly recommend using a weighing scale and measuring out your ingredients the metric way. I have provided a conversion to US cups but please note that I haven’t tested the recipe with this method. Just click on ‘US Customary’ above the ingredients to see the cup measurements. I make sure that all of my ingredients are at room temperature before I start the recipe, so pull out all of the ingredients before you start baking.Butter: I use European style butter in my recipes. This will make your frosting much creamier and richer.Salt: If you don’t have kosher salt, halve the amount of salt called for in the recipe. 1 teaspoon kosher salt = ½ teaspoon fine salt. Please note, I'm using Diamond Crystal Kosher Salt and not Morton salt, which is far too salty. Buttermilk: If you don’t have buttermilk, replace it with an equal amount of sour cream or full fat yoghurt. You can also make your own buttermilk, for every cup of buttermilk add 1 tablespoon of lemon juice to 1 cup of milk and let sit for 5 minutes. Vegetable Oil: Use an unflavoured oil like canola oil or sunflower oil. Round Pan: want to make this in a round pan? Bake in a 9-inch round pan and bake for the same amount of time. Storage: Keep tightly wrapped in the fridge for up to 5 days. Let it come to room temperature before eating.