Pre-heat your oven to 180°C/ 356°F. Prep your 6 inch baking tins with a circle of baking paper at the bottom. I use a bit of oil to make the paper stick to the tin.
In a large bowl, combine the milk and the apple cider vinegar. Put this to the side and allow it to curdle for a couple of minutes. Then add the oil, caster sugar and vanilla extract. In another bowl, sieve the flour, baking powder, soda + salt and combine. Using a large spoon, add the dry mixture to the wet, whisking as you go.
Pour your cake batter into 2 lined tins, evenly. Bake for 50 minutes, or until a knife comes out clean. You will need to bake this for less if you are using wider tins. For example, if you are using two 8 inch tins, only bake the cakes for 30 minutes.
Once your cake is completely cool, slice the cake into four even layers. I use a cake leveller to do this as it is WAY easier than using a knife. To use this, set your leveller to the height you want your cake to be, and then use it to cut your sponges.
In a standing mixer, place your butter and mix on a high speed for at least 5 minutes. This will cream your butter which will remove some of its colour and prepare it for the buttercream. Add all the remaining icing ingredients and mix on a low speed for around 30 seconds and then increase to a high speed, until combined and fluffy. Leave this mixing until it reaches your desired colour/ texture. Add 1 tablespoon of water at this point if you think the icing needs to loosen up a little bit.
Using a cake plate, layer all of the sponges up evenly with icing. Within each icing layer, create an icing barrier around the edge and place approx 1 tablespoon Biscoff spread in the middle. Piping bags make this easier.
Using the piping bag with icing in, spread the icing around the edge and the top of the cake. Use a cake spatula to smooth all out. If you don’t have these, you can do more of a rustic pattern with a knife. Place the cake in the fridge to cool for as long as possible (ideally at least a couple of hours).
Place 3 tablespoon Biscoff spread into a bowl and microwave for around 20 seconds, until it is melted. Add this to a small piping bag and cut a tiny hole at the edge of the piping bag, to drip onto the cake. Carefully drip the spread onto the edge of the cake, with around a 1cm gap between each drip. Add a bit more spread to some drips, enabling a mix of long and short drips on the cake.
Decorate the top of the cake in whatever way you want. I did icing swirls and then halved the 6 Biscoff biscuits and scattered them in between the icing swirls. ENJOY!