Prepare cake as per recipe. Once cool, cut layers in half horizontally. I used a cake leveler to do this.
Optional, but will add more moistness to your cake.
Place water and sugar into a small pot over high heat. Stir until sugar has dissolved. Remove from heat and cool completely. Store in fridge until needed.
Swiss Meringue Buttercream:
Place egg whites and sugar into the bowl of a stand mixer, whisk until combined.
Place bowl over a hot water bath on the stove and whisk constantly until the mixture is no longer grainy to the touch (approx. 3mins).
Place bowl on your stand mixer and whisk on med-high until the meringue is stiff and cooled (the bowl is no longer warm to the touch (approx. 5-10mins)).
Switch to paddle attachment.
Slowly add cubed butter and mix until smooth. Add vanilla.**
Cut cake layers in half horizontally using a cake leveler or large serrated knife.
Place one layer onto your cake stand, brush with simple syrup. Add 1/4 of the buttercream on top and spread roughly. Spread 1/3 of sliced berries over top of the buttercream.
Repeat the process with the other layers, finishing with 2 cups of whole berries.
* Ensure there is NO trace of egg yolks in your whites and that your mixer bowl and whisk is completely grease free or your meringue won't stiffen ** The buttercream may look like it's curdled at some point. Keep mixing until it is completely smooth.