1 1/2 tsp boiling water (from recently boiled kettle)
Dulce de leche
Simple syrup, to brush
With an electric hand mixer, whisk the egg whites until soft peaks start to form. Add a pinch of salt, then add the caster sugar gradually - mixing thoroughly after each spoonful.
Once the sugar is incorporated, whisk the mixture to firm peaks.
Carefully sieve in the almond and icing sugar mix, then fold this through, taking care not to knock the air out of the egg whites. Make sure there are no big air bubbles or lumps of egg white.
Once mixed together, put the mixture into a piping bag fitted with a 1cm nozzle.
Pipe 3.5cm rounds onto the a baking sheet lined with non-stick baking paper (leave a few cm between each one - for perfectly sized macarons you can use a template underneath the baking paper).
Slam the macaron tray down on the counter a few times - to help shape them and get rid of any bubbles, then let them sit for around an hour, so that a dry skin forms on the tops.
Preheat the oven to 120C, then cook the macarons for 15-18 minutes.
Remove the trays from the oven and immediately slide the paper off the hot trays. Let the macarons cool completely on the paper.
For the buttercream, process the icing sugar and custard powder to get rid of any lumps.
Add the softened butter to a large bowl and using an electric hand mixer whisk until completely soft. Add the icing sugar and custard mixture to the softened butter in large spoonfuls, beating in between additions until you have a smooth buttercream. Add the vanilla and boiling water and beat a final time to combine everything.
To assemble, remove the macarons from the parchment paper. Lightly brush the top of a macaron with simple syrup, then immediately dip into the granulated sugar. Place the macaron on the bottom of a sheet pan or baking pan and torch until the sugar has caramelized. Repeat with the remaining half of the macarons.
Transfer the buttercream to the pastry bag with a small round tip. Pipe a ring of buttercream, not quite reaching the edge of the macaron. Fill the hole with dulce de leche. Top with a second macaron and gently press to spread the buttercream to the edges. Repeat with remaining macarons and filling.