Prepare the blackberry syrup. Put all the ingredients in a saucepan and bring to a simmer over low heat, stirring so the sugar dissolves. Cook for a few minutes, until the blackberries start to burst and release their juices. Strain using a fine sieve, collecting the syrup into a bowl.
Preheat the oven to 180C (350F). Spray three 15cm (6in) cake tins with cake release spray and line the bottoms with greaseproof paper. The layers will be quite high as there's enough batter for four 15cm (6in) layers. If making four layers adjust baking time to 25-28 minutes.
Add the flour, sugar, cocoa powder, baking powder, bicarbonate of soda and salt to the bowl of your food processor. Pulse for a minute on low speed to combine.
Add the cubed butter and mix for 2-3 minutes until the mixture resembles coarse sand.
Add the eggs and mix for a minute until well incorporated. Use a spatula to scrape the bottom and sides of the bowl to make sure everything is mixed in well.
Mix the milk and vanilla extract in a measuring jug. Gradually add to the batter, mixing on low speed until combined. Add the honey and mix until you have a smooth glossy batter, scraping the bottom and sides of the bowl as needed.
Divide the batter between the prepared tins and bake for 35-40 minutes, until the cakes are firm on top and a skewer inserted in the centre comes out clean.
Cool the cakes in the tins for 10 minutes then turn out to a wire rack to cool. Once cool, level the tops, if needed.
Make the buttercream. Put the sugar, honey and water in a deep saucepan stir to combine. Bring to the boil over high heat and check the temperature with a digital or jam thermometer. Cook until it reaches 120°C/ 248°F.
Meanwhile, put the egg whites in the bowl of a stand mixer fitted with the whisk attachment. Whisk at maximum speed until you have soft peaks.
Carefully pour the sugar syrup into the egg whites, while mixing, down the side of the mixing bowl. Continue to whisk for several minutes until you have a glossy meringue and the bowl feels barely warm to the touch.
Add the butter, a few chunks at a time, and continue whisking at maximum speed until it is all incorporated. The buttercream may briefly separate or look curdled, but keep whisking and it will soon become glossy and smooth.
Add the blackberry syrup and mix it in. For a deeper pink colour you can add a little pink food colouring until you have the desired shade.
Brush the cake layers with any leftover blackberry syrup. Sandwich together with the buttercream, piping or spreading it over the layers.
Pipe a generous amount of buttercream on top of the cake – I used a Russian ball piping tip to create the ruffles.
Carefully drizzle any remaining blackberry syrup over the top of the cake, letting it drip down the sides. Serve immediately.