Preheat the oven to 180C (160C fan) / 350F. Grease the sides of two 23cm / 9 inch loose-bottomed tins and line the bottoms with non-stick baking paper.
In a large mixing bowl, whisk together the milk, coffee and vinegar, then set to one side.
In a separate bowl, add the dry ingredients (plain flour, wholemeal flour, baking powder, bicarbonate of soda and salt) and stir to combine.
Add the oil, sugar and vanilla extract to the wet ingredient bowl, and whisk until fully combined (using a hand whisk). Sieve in a third of the dry ingredients and whisk to combine. Add another third and whisk to combine. Add the last third along with any remaining wheatgerm left in the sieve. Stir through to combine (but do not over stir).
Fold in the walnuts.
Divide the batter equally between the prepared tins. Place on racks as close to the centre of the oven as possible. Bake for 40-45 minutes, until a skewer inserted into the middle comes out clean and the the cakes are springy when lightly pressed. (In my oven, I find the cake on the higher rack is ready in 40 minutes, and the cake on the lower rack often needs an extra 5 minutes).
Leave the cakes in their tins for at least an hour (they will firm up as they cool), then transfer to a wire rack.
While they’re cooling, prepare the frosting. Place the icing sugar, dairy-free spread/vegan butter and coffee in a mixing bowl. Stir with a metal spoon until it starts to combine, then beat with electric beaters (or a fork) until light and fluffy. If the frosting looks a little stiff, add a teaspoon of water and keep beating. Store the frosting in the fridge until needed.
Once the cakes are completely cooled, decorate with the frosting and assemble. Sprinkle on your toppings of choice and lightly press them into the frosting. Chill the cake before serving.
Stored in an airtight container, the cake will keep for about three days at room temperature, and up to a week in the fridge. It also freezes well.