This is from London Eats blog. I would simply leave a link to it but I am scared the site will be removed and I will be left without it! So all credits to this blogger, but due to the fabulous outcome I am shamelessly copying it here.
London Eats suggests that making smaller batches leaves less tolerance on your measurements. I whole-heartedly-agree, as my previous 3 attempts involved smaller and smaller batches in order to waste less. My first attempt using the recipe below exactly was a success.
This took me about 3 hours, all up. But it was worth it. I made them for my family at Easter and every single person raved about them, without fail.
To make salted caramel macarons (makes 25-30):
For the shells:
• 175g icing sugar
• 175 ground almonds
• 130g egg whites (about 4 eggs), at room temperature
• 175g light muscovado or brown sugar
• 75ml water
• caramel food colouring
For the filling:
• 150g white sugar
• 50ml water
• 180g salted butter (divide into 30g and 150g)
• 150ml double cream
• vanilla extract
• salt, very finely ground
1. Preheat the oven to 150°C (300°F). Line two large baking trays with greaseproof paper.
2. Mix the icing sugar and ground almonds, and push through a fine sieve. Put into a large bowl.
3. Divide the egg whites into two portions (2 x 65g). Add one half to the almond/icing sugar mixture and mix until you have a smooth, thick paste.
4. Next, make an Italian meringue. Put the muscovado or brown sugar into a saucepan. Add caramel/brown colouring as desired (I used enough to enhance the brown tint from the sugar, probably 20 drops of water-based colour) to the water and add the sugar in the saucepan. Heat to 114°C (237°F). In the meantime, whisk the rest of the eggs whites until frothy. Acting quickly, pour the hot syrup into the frothy eggs and beat the living daylights out of them! The mixture should quickly start to turn pale and fluffy, and increase in volume. Whisk for 5 minutes until the mixture is stiff and glossy – it should easily hold its shape.
5. Take one-third of the meringue mixture, and fold into the almond paste mixture to lighten it. Fold in the next third, then fold in the final third. Try to do this gently, and don’t mix too vigorously or for too long.
6. Fill a piping bag fitted with a 1 cm hole nozzle. Pipe out the macarons, leaving a few centimetres between each. Leave to dry at room temperature for around 20 minutes.
7. Bake the macaron shells for around 12-15 minutes until the shells have developed little feet but they are not browned. You might want to open the door briefly during baking to let any steam escape. If your oven cooks unevenly, make sure you turn them around halfway through. When baked, remove from the oven, allow to cool, then peel from the baking sheet. Arrange on a cooling tray and prepare the filling.
To make the filling:
8. Put the sugar and water into a small saucepan. Place on a medium heat until the mixture turns into a medium golden caramel (don’t be tempted to stir it at any point – it will turn into a crystallised mess!). The colour should be rich but without any burnt or acrid smell.
9. Remove the saucepan with the caramel from the heat, add the 30g of butter and stir well. It will sizzle, so watch out! Add the cream and vanilla to taste (just a drop or two) and stir until smooth. Put the pan back on the heat, and cook until it reaches 108°C (225°F). Remove from the heat and leave until almost cooled.
10. Put the cooled caramel and 150g 0f soft butter into a bowl and beat with an electric mixer until perfectly smooth. It might seem like the mixture has curdled at one point, but keep going and it will come good. You should end up with a very smooth cream. Add a dash of powdered salt (to taste, but go a little at a time).
11. Fill a piping bag with the salted caramel cream and use to fill the macarons.
12. Leave the macarons in the fridge for 24 hours, and remove from the fridge a couple of hours before serving.