Preheat your oven to 175°C (350°F). Line two 6" cake tins with parchment paper and set aside. Add the walnuts to your food processor and blitz until finely ground. Alternatively, you can use a knife to chop them finely.
Mix the instant coffee with a ¼ cup of boiling water and set it aside to cool for 5 minutes. In a glass, mix together the soy milk and apple cider vinegar and allow to sit for 5 minutes. To a large bowl add the coffee, soy milk mixture, rapeseed oil, and vanilla extract and whisk to combine.
Add the flour, sugar, baking powder, salt, and walnuts to the bowl with the wet ingredients, gently folding the mixture each time. Be careful not to over-mix, a few pockets of the flour are fine.
Add the cake batter to the prepared cake tins and bake for 40-45 mins. Check the cake is ready by inserting a knife or toothpick into it, which should come out clean.
Allow the cakes to cool in the pans for about 20 minutes before carefully transferring them out onto a cooling rack. Allow them to fully cool before transferring them to the fridge in an airtight container until ready to use.
To assemble the cakes, first, use a serrated knife to cut off the domes if there are any. Note: you can leave the dome on the top piece if preferred.
Pipe some buttercream on one of the cakes and place the second cake on top. Coat the entire cake with buttercream or just frost the top of the cake as preferred.