Sift the icing sugar, almonds and rosemary into the food processor and blitz to give yourself an even finer powder.
In a large mixing bowl, whisk the egg whites to a foam using an electric mixer then add the caster sugar gradually and continue beating until you have a meringue that stands in soft peaks. Add the gel food colouring on the end of a cocktail stick and then continue beating once more until you get stiff peaks and the colour has been mixed throughout.
Tip all the icing sugar, almonds and rosemary on top. Using a silicon spatula, sweep around the bowl in a circle and then cut sideways strokes with the thin blade of the spatula through the centre backwards and forwards ten times. Repeat sweeping around the edge of the bowl and doing your ten strokes five times so that you've done fifty strokes. Your batter should be roughly ready by this point, you are looking for a flowing lava effect. If it is too stiff continue sweeping around the edge of the bowl and doing another ten strokes until you are happy with the flow. (This recipe seemed to require a lot more strokes than usual... Maybe 80... I’m not sure why this happened today)
Fill piping bags with the batter, I use disposable ones with around 1.5cm width snipped of the end. Fix parchment paper to your baking sheet with a blob of meringue batter in each corner.
Pipe discs in a circular movement around the size of a two pound coin (4cm). Allow a similar distance between the piped circles incase they spread. Pick the tray up with both hands and rap on the table firmly to bang any air bubbles out and make the circles of batter settle.
Preheat the oven to 150c. Leave the piped circles near a radiator for 20 minutes to dry out (winter only). In summer, leave for 20-30 minutes. The surface of the circles should dry out so that they are no longer sticky to the touch. The feet develop as the surface has toughened before the centre has cooked, the pressure that builds up under heating forces the top of the macaron to rise, then you should get feet.
Bake for 12-18 minutes depending on size. The length of time really is trial and error. I put mine on the lowest oven shelf but again you will need to experiment. And everyone's oven is different - never more obvious than with macarons!
Hopefully, if you've cooked them enough but not too much, you'll have that happy medium of a surface that peels beautifully off the baking parchment but a meringue which remains soft inside. If you are having trouble removing them from the paper, some drops of water sprinkled under the parchment whilst still warm will help steam the macarons off. But I find that they come off best when completely cool and don't need this. So don't be impatient!
When cool, spread your filling on the flat side of a shell and sandwich with another, squeezing gently. Allow to set for a couple of hours. I find they keep in an airtight tin for a week. If you can resist them.
Notes
This should yield 18-24 pairs of macaron shells - depending on air volume and how big you pipe them, it's hard to give an exact figure. Naturally the calorie content will be less if you end up with more smaller ones, and it's only a guide anyway. The ingredients for the shells have been adapted from Ottolenghi, The Cookbook, “Lime and Basil Macarons” although my method departs from theirs in that I’m from the “beat it to death” school rather than softly softly. The buttercream was leftover lemon cupcake frosting. This quantity of buttercream makes enough to cover 12 cupcakes so you will need to scale the amounts down if you only want a small amount to fill macaron shells.