Vegan vanilla sponge layers sandwiched and covered with creamy dairy-free Biscoff frosting.
Preheat your oven to 180°C/350°F (160°C/325°F if using a fan or convection oven). Grease, flour and line 3 7-inch cake pans.
Mix together the soy milk and the vinegar. Whisk briefly with a fork and set to one side- this will become "curdled" and will act as vegan buttermilk.
In a large bowl, mix together the flour, sugar, baking soda and cinnamon.
Melt the butter in the microwave for 20-30 seconds, or until there are no lumps left, and leave it to cool for a few minutes.
Add the melted butter to the bowl along with the buttermilk from earlier and the vanilla extract. Mix everything until well combined.
Add the crushed Biscoff biscuits and fold the crumbs into the batter.
Divide the cake batter between your 3 lined tins and bake for 15-18 minutes, or until a toothpick through the middle comes out clean.
Leave the cakes in the tins for 10 minutes then flip them out onto a wire rack and let them cool completely.
Using an electric whisk or stand mixer, beat the vegan butter in a large bowl until soft and fluffy. Add in the Biscoff spread and whisk again until combined.
Add in half of the icing sugar, a little at a time, whisking in between.
Add the rest of the icing sugar and the plant-based milk. Whisk until it becomes soft and fluffy.
Spread a thick layer of the Biscoff frosting onto the first cake using a palette knife. Place the second cake on top and repeat until you have a stack of 3 iced cakes.
Spread some frosting down the sides of the cake and use a scraper to smooth it out and make a crumb coat. Place the whole cake in the fridge for 1 hour, then repeat so the cake has 2 layers of frosting in total.
Place 6 of the Biscoff biscuits on the cake, standing them upright in a circle (as shown in pictures). Pipe some swirls around the biscuits using the leftover frosting.
Melt the Biscoff spread in the microwave for 10-20 seconds and drizzle it over the top of the cake.
Crush the remaining 2 biscuits into crumbs and press the crumbs into the bottom edges of the cake.