Preheat oven to 180°C. Grease the base and sides of three deep, round 15cm cake pans and line the bottoms with baking paper.
To make the cake, sift together the flour and baking powder into a large bowl and set aside. Mix the milk and elderflower coridal in a small bowl and set aside. Using a stand mixer or electric whisk, beat the butter, sugar and lemon zest for 5 minutes, or until pale and fluffy. Add the eggs one at a time, beating well after each addition. If the mixture looks like it's starting to curdle, add 1 tablespoon of the flour mixture. Turn the mixer speed to low and add half of the flower mixture followed by the mix and elderflower. Stir in the remaining flour and divide the batter equally into the prepared cake pans. Bake the cakes for 35-37 minutes, or until a toothpick inserted in the centre comes out clean. Let the cakes cool in their pans for 10 minutes before turning them out onto a wire rack to cool completely. Once cool, wrap them tightly in plastic wrap and place in the fridge to firm up.
To make the puree, preheat the oven to 190°C. Slice the steams off the strawberries, cut them in half and put them into a baking dish. Toss the fruit with the sugar and vanilla. Rost the strawberries in the oven for 30-40 minutes, stirring once halfway. The strawberries are done once they are soft and fragrant and the juices have turned into a syrup. Leave the strawberries to cool completely before transferring them to a food processor or blender and pulsing until smooth. Store in an airtight container in the fridge until you're ready to use it.
To make the butter cream, add the egg whites and sugar to the bowl of a stand mixer or a heatproof bowl. Make sure there are no traces of grease in the bowl as this will stop the meringue from whipping up. Place the bowl over a saucepan of simmering water, making sure that the base doesn't touch the water. Whisk the eggs and sugar constantly until the mixture reaches a temperature of about 150°C. If you don't have a thermometer, rub a little of the mixture in between your fingers. The eggs should be hot to the touch with all the sugar dissolved.
Remove the bowl from the heat and start whisking the egg whites on high speed in a stand mixer fitted with the whisk attachment or with an electric whisk. Beat until the whites are thick and have doubled in volume. Once the bowl is cool to the touch and with the mixer still running, add the butter a piece at a time. The meringue will delfate and will start to look a little runny or curdled. Don't panic - this is normal. Continue beating until you have added in all of your butter and the mixture is smooth and glossy. If after 10 minutes, your meringue buttercream is still soupy, chill it in the fridge for 20 minutes and beat again. Add about 125g of the strawberry puree and beat the buttercream for 2 minutes or until smooth. Leave the buttercream at room temperature until you are ready to assemble.
Unwrap the cakes from the fridge and if any of them are domed, level them with a serrated knife or a cake leveler. Add the laters need to be completely flat to ensure you don't end up with a wonky cake. Brush each later with 30ml of elderflower cordial. Place one layer of cake on a cake board and put this board on a turnable, if using. If you buttercream has firmed up, whip it again in the mixer until smooth. Add a scoop of buttercream and spread it out evenly with an offset spatular, pushing the buttercream right to the edge (it's fine if it spills over). Place the next layer on top of the first, but this time add about 1 tablespoon of strawberry puree on top of the buttercream layer, leaving about 2.5cm clear around the edge. Place the last layer top side down to give you a perfectly flat top.
Frost the top and sides of the cake with a thin layer of buttercream. Smooth the edges as much as you can with a bench scraper or palette knife. This is the crumb coat layer that will trap any stray crumbs and act as a base for the next layer. Place the cake in the fridge for 20 minutes to firm up before adding another layer of buttercream on the top and sides. Fill a piping bag fitting with an open star nozzle (I've used Wilton 8B) with any remaining buttercream and pipe swirls acorss the edge to make a wreath. Top the cake with fresh strawberries, a drizzle of any leftover puree and fresh elderflower (if in season.
