Preheat the oven to 180℃/160℃ fan/350℉/gas mark 4. Line two 20 cm/8 in round cake tins with baking paper.
Whisk together the Biscoff spread and the melted butter in a large bowl to loosen the spread up. Whisk in the oil and sugar, followed by the milk, vinegar and vanilla extract.
Sift in the flour, cornflour, baking powder, bicarbonate of soda, cinnamon and salt. Stir with a balloon whisk until no dry lumps remain but do not over-beat the batter.
Divide the mixture evenly between the prepared cake tins and spread it level. Bake for about 25-30 minutes until a skewer inserted into the centre comes out clean.
Leave the cakes to cool in the tins for 15 minutes then carefully turn them out onto a wire rack and leave to cool completely.
To make the buttercream, whisk together the Biscoff spread and the softened butter with an electric mixer until well combined. Sift in the icing sugar and whisk for a good few minutes until very light and fluffy.
Trim the tops of the cakes to level them then spread a layer of buttercream over the top of each cake. Sprinkle one of the cakes with crushed Biscoff cookies, keeping them away from the edges.
Place the other cake layer on top, buttercream side down.
Use a palette knife to cover the top and sides of the cake with buttercream. Place the cake in the fridge to chill for 10-15 minutes.
Gently melt the 100g of Biscoff spread in bursts in the microwave until it is runny but not hot; allow it to cool slightly.
Use a spoon or small squeezy bottle to drizzle the Biscoff around the top edge of the cake; use the back of a spoon to gently encourage it to run over the edge then fill in the middle. Place in the fridge to set.
Transfer the remaining buttercream into a piping bag fitted with a large star nozzle. Pipe it on top of the cake as you like then decorate with whole and/or crushed Biscoff cookies.