Stir the water and sugar together in a small pot over high heat. Bring to a boil and do not stir again. Heat the sugar to 238℉ (soft ball stage).
While the syrup is cooking, whip your eggs and egg yolks until they are pale and light.
When the syrup is ready, turn the mixer to low and very, very slowly add the syrup to the eggs. Allow it to drip down the sides of the bowl into the eggs.
Increase the speed to medium and whip the mixture until the bowl is cool to the touch. This may take 5 minutes. Do not rush it.
Reduce the speed of the mixer back to low and slowly add in the butter one Tablespoon at a time. Once all the butter has been added, increase the speed to medium and whip for another 5 minutes. It'll get pretty ugly before it gets beautiful.
Finally add the salt and get the rest of the cake ready to be frosted.
Assemble the Cake
Divide the buttercream into two 3 cup bowls. Cover one bowl set to the side; add ½ cup of the lemon curd to the second one. Mix the lemon curd and frosting together. Add ½ cup of the lemon buttercream into a pastry bag.
Place one of the cakes on a cake round or directly on to a cake platter. Brush on, or use a squirt bottle, about ⅓ cup of the lemon simple syrup. Be generous.
Spoon 1 cup of the lemon buttercream on to the cake and smooth out. Add a frosting dam around the cake then fill in with ½ cup of lemon curd. Add in a few raspberries. Top with second cake.
Repeat with the syrup, frosting, dam, curd and more raspberries. Top with the third cake upside down so that the edges are squared off. Add the simple syrup to the cake then add a thin crumb coating with any lemon buttercream remaining. Place into the fridge to firm up for 1 hour.
Once it has firmed up, frost the cake with the reserved plain frosting. You may have to beat it a wooden spoon to smooth it out again if it separates. Frost and decorate as desired. I kept it basic and simple.
Top the cake with ⅓ cup of lemon curd and a few pretty raspberries.
Store in an airtight container in the fridge for up to 2 days (if it lasts that long).