Add egg whites and sugar to a heatproof bowl. Place the bowl over a saucepan with boiling water. Make sure the bowl doesn’t touch the water. Cook for about 3 - 4 minutes, whisking gently with a whisk until the mixture reaches temperature 65°C / 150°F.
melt the chocolate
Melt the chocolate. Set aside to cool to room temperature. Remove the swiss meringue mixture from the saucepan when it reaches the needed temperature and continue mixing with an electric mixer (or stand mixer) for about 10 minutes or until the peaks are nice and stiff. Transfer half of the swiss meringue to a clean bowl.
add cold butter to swiss meringue
Gradually add small cubes of cold butter to one half of the swiss meringue. Whip on the highest speed with an electric mixer. Only add the next cube of butter once the first one has been completely incorporated. If the mixture starts to look curdled, don’t worry, just continue whipping, it will come back together.TipIf we added butter to the swiss meringue when it was still too warm, it can happen that the mixture becomes liquidy. If this happens, place the mixture in the fridge for about 10 minutes, then continue whipping. In this case the mixture won’t be as nice and airy, but it will be still creamy and delicious.
Chocolate Swiss Meringue Buttercream
Add the melted and cool chocolate to the swiss meringue mixture. Using an electric mixer mix well to get a nice, smooth mixture. Then, using a spatula gradually incorporate the rest of the swiss meringue (egg whites + sugar). Place in the fridge for 10 minutes.
Fill the waffle cones
Transfer the Chocolate Swiss Meringue Buttercream to a pastry bag or you can use an ice cream scoop. Fill the waffle cones with the buttercream. Place in the fridge for 2 hours or in a freezer for 1 hour.
Dip in chocolate and serve
Melt the chocolate, add the vegetable oil and stir to combine to get a nice, smooth chocolate mixture. Dip each top scoop in the chocolate to cover the top. Leave any excess chocolate drip off, then immediately transfer to a tall glass and transfer to a fridge to chill. Optionally decorate with sprinkles, chopped hazelnuts before placing it in the fridge. Half an hour before serving, place the Chocolate Swiss Meringue Buttercream Filled Cones to room temperature, then serve.