Make the cake. Preheat the oven to 180C (350F). Lightly spray three 15cm (6in) cake tins with cake release and line with baking paper.
Add all the dry ingredients (flour, sugar, baking powder/soda and salt) to the bowl of a food processor. Pulse briefly to mix together.
Add the cubed butter and pulse again until the mixture resembles breadcrumbs.
Mix the eggs and vanilla together. Gradually add the eggs through the feeding tube and process for a minute until well incorporated.
Gradually pour the milk into the processor through the feeding tube and process together until you have an evenly mixed batter. Stop the mixer and use a spatula to scrape the bottom and sides of the bowl to make sure the batter is thoroughly mixed.
Divide the batter between the tins (each will be around 210g/ 7.4oz) and bake for 20-22 minutes until the cakes are springy to the touch and a skewer inserted in the center comes out clean.
Leave to cool in the tins for 10 minutes then turn out to a wire rack.
Brush the sponges with a little sugar syrup while they are still a little warm and leave to cool completely.
Wrap well in plastic wrap and chill in the fridge, ideally overnight. Level the cake layers, if needed, using a large serrated knife before frosting the cake.
Make the frosting. Put the butter and icing sugar in a stand mixer fitted with the paddle attachment. Beat well, starting on low speed setting and increasing to maximum speed once ingredients are incorporated.
Keep beating until light and fluffy, scraping the bottom and sides of the bowl halfway through.
Add the vanilla paste and jam and beat until smooth.
Transfer a third of the buttercream into a large piping bag fitted with a plain round tip.
Add about 10 tablespoons of jam into a smaller piping bag fitted with a plain round tip.
Pipe a ring of buttercream around the edge of the bottom cake layer.
Pipe or spread jam inside the ring and spread to fill.
Pipe more buttercream on top of the jam and smooth.
Top with the second layer and repeat the process.
Pipe or smooth a thin layer of buttercream (crumb coating) over the entire cake. Use a palette knife to smooth the top of the cake letting the buttercream spread over the edges.
Use a side scraper to smooth the buttercream on the side of the cake – using a turntable will make this easier.
Freeze the cake for 30 minutes before applying a second layer of buttercream to cover the cake. Repeat the process to smooth the buttercream for a clean finish.
Pipe any extra buttercream on top of the cake using a selection of small or large star tips.
Decorate with a few fresh blackberries and some edible flowers if you like. Add a little gold dust or pastel sprinkles and serve.
Video
Notes
Make the sugar syrup: Put 100g each of sugar and water in a saucepan. Stir together and gradually bring to a simmer over medium heat. Take off the heat as soon as small bubbles appear on the surface. Add 1 tablespoon of vanilla paste and cool completely before using. The syrup will keep up to a month in the fridge. Use it to brush over cakes to keep the sponge moist. Make the cake in a stand mixer: add the dry ingredients and mix briefly with the paddle attachment. Add the butter and mix until the batter resembles sand. Mix in the eggs, scraping the bowl as needed, and finally beat in the milk until batter is smooth.