Juglans nigra, the black walnut, is not a polite nut. Where the Persian walnut is mild, buttery, and accommodating, the black walnut is muscular and demanding: intensely aromatic, slightly musky, with a depth of flavour that can fill a kitchen the moment the shell cracks. This intensity is produced by juglone and a high concentration of aromatic compounds that make the black walnut unlike any other nut in the world. It is the walnut of the American eastern woodland; native from the Great Lakes to the Gulf Coast, gathered by indigenous peoples for millennia, adopted enthusiastically by European settlers, and woven into the baking tradition of the Appalachian communities of Virginia, West Virginia, Kentucky, and Tennessee. The black walnut cake is the flagship of this tradition: a layer cake in which the walnut's power is displayed rather than tamed, paired with brown butter frosting whose caramelised nuttiness echoes and amplifies the walnut rather than competing with it. The cake originated in the farmhouse kitchens of the Shenandoah Valley and the Blue Ridge piedmont, where black walnut trees grew in every dooryard and the autumn harvest of cracking and shelling nuts was a community activity: a laborious one, since the black walnut's shell is among the hardest of any nut in the world, and the husks permanently stain everything they touch deep brown. The resulting kernels, once freed, reward the labour with a flavour unmatched by anything imported.
Ingredients
Cake
250 g plain flour
1.5 tsp baking powder
0.5 tsp baking soda (bicarbonate of soda)
0.5 tsp fine sea salt
200 g caster sugar
100 g unsalted butter, softened to room temperature
3 large eggs, at room temperature
240 ml buttermilk, at room temperature
1 tsp pure vanilla extract
200 g black walnut halves, toasted and roughly chopped (reserve 10–12 whole halves for decoration)
Frosting
170 g unsalted butter
350 g icing (powdered) sugar, sifted
60 ml heavy cream, plus more as needed
1 tsp pure vanilla extract
1 pinch fine sea salt
Method
Preheat oven to 175°C (350°F). Grease and flour two 20cm (8-inch) round cake tins. Toast the black walnuts in a dry frying pan over medium heat for 3–4 minutes until fragrant. Cool completely, then chop roughly.
Whisk together the flour, baking powder, baking soda, and salt in a bowl. In a separate large bowl, beat the softened butter and sugar with an electric mixer on medium-high speed until pale and very fluffy; at least 4 minutes. Beat in the eggs one at a time, mixing well after each addition. Beat in the vanilla.
Reduce mixer speed to low. Add the flour mixture in three additions, alternating with the buttermilk in two additions (flour, buttermilk, flour, buttermilk, flour), mixing until just combined after each. Do not overmix. Fold in the chopped black walnuts by hand with a spatula.
Divide the batter evenly between the prepared tins. Bake for 30–35 minutes until a skewer inserted in the centre comes out clean and the top springs back when lightly pressed. Cool in tins for 10 minutes, then turn out onto a wire rack and cool completely.
Make the brown butter frosting: place the butter in a light-coloured saucepan over medium heat. Cook, swirling occasionally, until the butter melts, foams, and then the foam subsides and the milk solids at the bottom turn a deep golden brown and smell of hazelnuts; 6–8 minutes. Pour immediately into a large mixing bowl (including all the brown bits from the bottom of the pan) and cool in the refrigerator until solid, about 1 hour.
Beat the solidified brown butter with an electric mixer until fluffy and lightened in colour. Add the icing sugar in two additions, beating on low then high speed. Add the cream, vanilla, and salt. Beat until the frosting is smooth, fluffy, and spreadable. Adjust consistency with more cream or icing sugar as needed.
Place one cake layer on a serving plate. Spread a generous layer of frosting on top. Place the second layer on top and frost the top and sides of the cake. Decorate the top with the reserved whole black walnut halves.
Notes
Black walnuts are available online and from specialist American food suppliers. The shelled kernels are sold in vacuum-sealed bags. They can be frozen for up to 6 months without loss of quality. If black walnuts are genuinely unavailable, the cake works with Persian walnuts; the flavour is milder but still excellent. The brown butter frosting is not optional; its caramelised depth is the frosting specifically designed for this cake.
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