Thoroughly clean the whisk attachment and heatproof bowl of a stand mixer, and wipe with a paper towel dipped in lemon juice or vinegar to remove any traces of grease.
Combine the egg whites and sugar in the mixer bowl. Place the bowl over a pot of just-simmering water, making sure the water doesn't touch the bottom of the bowl. Stir constantly until the sugar has dissolved and the mixture reaches 71°C/160°F.
Place the bowl on the mixer and whip on medium speed for one minute, then increase speed to medium-high and beat until the meringue is stiff and glossy. The mixer bowl should be cool to the touch.
Switch to the paddle attachment, and with the mixer on low speed add the butter a piece at a time until fully incorporated. If the mixture seems to curdle or separate, don’t panic! Just keep the mixer going on low speed until the buttercream becomes silky and smooth.
Warm the caramel sauce gently in the microwave until just slightly warm and pourable (don't make it hot, or you'll melt your buttercream). Add to the buttercream along with the vanilla extract and beat again to combine.
Give it a taste, if it could do with more caramel then add a bit more.
Split your cakes into layers (I split each cake into two layers, so my finished cake had four layers), fill, layer and cover your cake with the buttercream.
See post for how I used the buttercream to decorate my cake.
Notes
If your buttercream does split and is taking a long time to come back together, you can speed the process up. Touch the side of the bowl - if it still feels very warm, then pop the bowl into the fridge for 5-10 minutes, then beat again. Alternatively, if it feels like the mixture is too cold, then place the bowl back over the pan of simmering water for a couple of minutes, just until it begins to melt around the edges of the buttercream, then remove from the heat and beat again.