Preheat your oven to 160°C (320°F) with the fan function on (refer to note 4 for conventional ovens). Prepare two 8-inch round cake pans by greasing and/or lining them. Set aside.

Sift the all-purpose flour, cornstarch, baking powder, baking soda, salt, and instant coffee powder into a bowl. Whisk to blend thoroughly and set aside.

In another mixing bowl, combine the softened butter, vegetable oil, and packed brown sugar. Using an electric mixer (either hand or stand), beat the mixture for about 2 minutes until it becomes creamy and fluffy.

Add the eggs, one at a time, beating each into the mixture for 10–15 seconds before adding the next. Stir in the vanilla extract, white vinegar, and ½ cup of buttermilk. Beat until the mixture is smooth, even if it looks slightly curdled this is normal.

Set aside your mixer and transition to folding by hand. Gently incorporate half the dry ingredients into the wet mixture using a spatula. Add the remaining cup of buttermilk, folding until just combined. Finally, fold in the rest of the dry ingredients, being careful not to overmix (refer to note 3). The batter should appear smooth and lump-free by the end (see note 7 if it remains curdled).

Divide the batter equally between the two prepared pans and bake for 30 minutes or until a toothpick inserted in the center comes out clean. Let the cakes cool in their pans for 20 minutes, then transfer them to a wire rack to cool completely before frosting.

Chop the white chocolate into small pieces unless you’re using callets. Place the chocolate in a heatproof bowl.

In a small saucepan, heat the cream over medium-low heat until it just begins to bubble gently. Remove the cream from the heat promptly and pour it over the chocolate. Add the instant coffee powder, but refrain from stirring just yet.

Let the mixture sit undisturbed for 5 minutes, then stir gently with a spatula before using an immersion blender to blend the ganache until smooth. Keep the blender submerged to minimize air bubbles.

Transfer the ganache to a wide, shallow dish for quicker cooling. Cover the surface with cling wrap to prevent a skin from forming and refrigerate for several hours until cold and slightly thickened. For best results, leave it to chill for 6+ hours or overnight.

When ready, whip the ganache with an electric mixer until it reaches semi-stiff peaks. It thickens quickly, so avoid overwhipping.

Melt the butter, brown sugar, and cinnamon in a saucepan over medium heat, stirring occasionally. Once the mixture starts bubbling, stir in the walnuts, ensuring they’re evenly coated.

Quickly remove the pan from the heat and spread the coated walnuts onto a tray lined with parchment. Use a spatula to separate them. Allow the walnuts to cool and harden for about 5 minutes.

If desired, level the tops of the cake layers with a serrated knife for a flat surface.

Place the first cake layer on a serving plate or cake stand. Spread a generous amount of whipped ganache frosting over the top, then sprinkle half the candied walnuts evenly.

Position the second cake layer on top and coat the entire cake, including the sides, with the remaining frosting. Use a piping bag fitted with a round tip (such as a 1A) for decorative swirls on top. Scatter the rest of the candied walnuts over the cake as a finishing touch.