Make the buttercream
For the buttercream, blitz 240g unsalted butter and 160g vegetable fat together in a food processor until smooth. Add 800g icing sugar and pulse until blended.
Spread a spoonful of buttercream into the middle of a cake board or turntable to stop the cake moving around. Place a cake on the serving plate and top with 100g of the buttercream. Spread with 1 tbsp of strawberry jam, then top with the second sponge, repeat and top with the third sponge.
Use 350g of the buttercream to crumb-coat the cake by applying a thin layer of icing around the entire side and top of the cake. Smooth and remove any excess icing with a palette knife. Chill for 15 mins until firm. Repeat this process to create a second, thicker layer of buttercream. Chill until firm.