Mix the cocoa powder and boiling water together in a large bowl until it forms a smooth paste, mix in the food colouring then add all of the other ingredients and beat with an electric mixer until it forms a smooth, thickish batter.

Divide the mixture between the tins and bake for about 25 minutes until a skewer inserted into the centre comes out clean, leave to cool in the tins for 10 minutes then transfer to a wire rack to cool completely.

Line a baking tray with a sheet of greaseproof paper.

Melt the dark and white chocolates in separate bowls over a pan of gently simmering water, do not allow the water to become too hot and don't get any in the chocolate.

Mix the powdered food colouring into the white chocolate a little at a time until you reach your desired shade.

Pour the dark chocolate onto the greaseproof paper and spread it out into an approx 15x25cm/6x10in rectangle, drizzle the white chocolate over the top and drag a toothpick through it to swirl the two colours together.

Set the chocolate aside until it has set but is still slightly soft then use a very sharp knife to cut it into triangular shards of varying sizes, place in the fridge until hard. Store in an airtight container.

Place the blackberries and sugar in a saucepan, bring up to a simmer and cook until the blackberries have broken down and become mushy. Pour into a blender and blitz until it becomes a puree then pass it through a fine mesh sieve into a bowl, cover and refrigerate until cold.

Place the egg whites and sugar in a spotlessly clean heatproof bowl (the bowl of your stand mixer if you have one) and place the bowl over a pan of gently simmering water, make sure that the base of the bowl doesn't touch the water.

Gently whisk the eggs (with a hand whisk, not an electric one) until the sugar has dissolved, if you rub the mixture between your fingers it should feel smooth with no hint of graininess. It should measure 60C/140F on a thermometer.

Remove the bowl from the heat and whisk with an electric mixer/stand mixer on high until the egg whites have formed a stiff, glossy meringue and the mixture is completely cold, the base of the bowl should feel cool to the touch, this will take about 10 minutes.

Turn the mixer down to low and gradually add the butter, a tablespoon at a time, mixing to incorporate after each addition. The mixture may curdle or begin to look soupy, just keep on whisking and it will come back together.

Once all of the butter has been incorporated and the buttercream is smooth and silky, whisk in the blackberry puree gradually, a spoonful at a time. The mixture shouldn't split, but if it does don't worry! Just place a couple of heaped tablespoons of the buttercream in a small bowl and microwave it until it has melted, then drizzle it back into the rest of the buttercream slowly while whisking on a high speed, after a couple of minutes of whisking it will come back together.

Finally whisk in the vanilla extract and add the food colouring a little at a time until you reach your desired shade.

Trim the tops of the cakes if necessary to level them then place one layer on a cake stand or serving platter, spread over a thinish layer of the buttercream. Place a generous dollop of the buttercream in a piping bag fitted with a large (1.5cm) plain nozzle; pipe a ring of the buttercream just inside the edge of the cake, this forms a dam to stop your jam from leaking out.

Fill the gap in the middle of the ring with a couple of heaped tablespoons of blackberry jam. Spread a thinish layer of the buttercream over the bottom of one of the other cake layers and place in buttercream side DOWN on top, press down gently. Spread a thinish layer of buttercream on top of the cake, pipe another ring of buttercream around the edge and fill with jam. Spread a thinish layer of buttercream over the TOP side of the remaining cake layer then place it buttercream side DOWN on to the cake so that the top surface of the cake is the base to create a completely flat surface, press down gently.

Crumb coat the cake by spreading a very thin layer of buttercream over the entire cake, don't worry about being too neat at this point, just make sure that the whole cake is covered. Place in the fridge for 15 minutes until the buttercream has set.

Spread a thick layer of buttercream over the whole cake, using an offset palette knife or plastic dough scraper to get it really smooth, take your time and fill in all the gaps but don't worry too much about getting it absolutely perfect. Place the cake in the fridge while you make the ganache.

Place the chocolate in a heatproof bowl, heat the blackberry puree and milk together in a small pan until boiling then pour over the chocolate and stir until smooth, add the butter and stir until it has melted.

Add the food colouring a little at a time until it reaches the desired shade. Due to the blackberry puree the ganache is a little thicker than usual and will not set as firm as regular ganache. If you find that it is too thick to pour so that it drips down the sides of the cake, stir in some warm milk a little at a time until it reaches a pourable consistency.

Pour the ganache over the top of the cake, using a palette knife or the back of a spoon to encourage it to drip over the edge and down the sides.

Allow the ganache to set a little then place any leftover buttercream in a piping bag fitted with a small star nozzle. Decorate the top of the cake with the chocolate shards and fondant roses, pipe blobs of the buttercream and sprinkle with edible lustre dust and black and purple sprinkles if you have them.